I found this topic to be well-described in The Ties That Bind: Peasant Families in Medieval England, by Barbara Hanawalt. Her book focuses on the farming serfs who made up the bulk of the population; the date range is the 13th to 15th centuries.
Regarding the size of houses, on pages 32-33:
At the lower end of the social scale was the hut of the cottar, the cottage. These were either one-room houses of about sixteen by twelve feet or possibly larger two-room houses of thirty-three by thirteen feet. The typical half-virgater or virgater ["middle class" for peasants] had a long-house. At one end was a byre that was usually separated from the living part by a cross-passage. The byre housed farm animals or other agricultural goods such as grain or farming, brewing, or dairying equipment. An internal passageway between the two parts of a long-house permitted access to either side, so that it had the appearance from the outside of being a long, low, continuous structure. Long-houses varied in size from buildings little larger than cottages to the more normal forty-nine feet in length; some were as long as eighty-two to ninety-eight feet.
An accompanying illustration shows larger houses as 15' across, so roughly 200 to 1,500 square feet, or 18 to 135 square meters. These were almost always a single level, but could have a loft for extra storage or sleeping space, referred to as a "solar" - rather different from a manor house's solar. Although a house was formally just one room, flimsy wattle-and-daub, wood, or stone partitions existed to separate animals, the cooking area, and sleeping areas.
Note that having livestock in your house was something to aspire to - they were stores of wealth, produced goods and services like wool, milk, or traction, and helped keep the house warm. You lived with the smell, and spent a lot of time outside. A typical modern barn would be an upgrade, even without aluminum siding, fiberglass insulation, or electric lighting.
Regarding construction, on pages 34-35:
...wood was the preferred building material. But it was so scarce by the fourteenth century that it was only used as a frame and the walls were filled in with turf in Devon and similar areas, cob in clayland, and wattle and daub through most of England....
The house frames were of two types. The cruck type was perhaps the older of the two. The cruck was a marvelous structure requiring considerable skill to make. A venerable tree, usually an oak, was cut and the trunk and the lowest branch split in half and shaped; or, possibly, two smaller trees were used. The two pieces were put together to form an arch, the branch portion providing the roof frame and wall portions and the other wooden parts all were constructed on the ground and then the whole cruck was raised into place. As the crucks were assembled they were raised in order, starting at one end.... By the fourteenth century trees large enough to serve as crucks were rare, so an alternative, the truss, was also used. In this construction posts made up the frame for walls and supported rafters for the roof. Tie beams held the structure together....
The cruck or truss made up the gable ends, the space between being called a bay. Cottages had only one bay, but the long-house might have two or more bays. Thus a house of two bays would have two gable ends and also an intermediate cruck in the center for support. Houses with more bays added another cruck. By the fifteenth century on the Worcestershire estates two- and three-bay houses were the most common, and even four-bay houses are mentioned.
This is why the houses were so consistent in width: they were built around the dimensions of the oak trees, rather than using cut timber in a modern "balloon" house, which can be any dimension (up to the practical limits of wooden construction, of course).
The roofs of the cruck and truss houses were usually thatched with straw and sometimes with rushes. Both types of frames left a natural hip that made thatching easy. Because there were no chimneys in peasant houses, the smoke exited directly through a hole in the thatch. To prevent the thatch from catching fire, tiles were placed around the opening. In some of the higher-quality houses, where appropriate materials were available, stone or slate roofs, were used in the fourteenth century.
The floors were usually of clay, although some stone, cobbles, and stone flags have been found. Wooden floors were very rare.... floors were covered with straw.
Stone construction was also used; the section is shorter, but it's not how common this material was relative to all-timber. Page 36:
Where building stone was readily available - and for the most part that meant on the site, for carting stone was expensive - houses began to be made of rough-cut stone in the thirteenth century. Thus, the stone houses at Wharram Percy were built of natural chalk that could be quarried in the village itself. Lacking mortar, which is found only in manor houses, the peasants filled the cracks with clay to keep out wind and water.... Yet another variation was the erection of a low wall of stone that kept out rising dampness; a timer structure was then placed on it with wattle-and-daub fillers. Carpenters and masons made little attempt to align squarely either stone walls or timber-frame houses, so that they seldom had square corners, straight walls, or doors opposite each other.
The cost of building a stone-and-timber house ran from 40 shillings for a cottage to 78 for a long-house in Devon in 1406; 10 to 30 shillings for a cottage in Northamptonshire around 1300; and 67 shillings for a cottage in Leicerstershire around 1600 (reflecting post-plague inflation).
One last note: these houses quickly deteriorated in the damp English climate:
...most houses were built to last one one generation.... When the old house began to disintegrate, a new one was erected.... Peasant sons apparently did not have strong attachments to an ancestral house, not even to the home of their fathers...
Though by the 14th century, "houses might be built to last forty or fifty years."
Doors with locks and windows with shutters (but not glass) rounded things out.