Well one way to answer would be to just show you some Roman maps. While they are not a perfect as modern maps, I would say that, for Europe at least, they are not bad. Remember that until recent times It was not possible to gauge longitude accurately. So for example, on this map copied in the 15th century from Ptolemy's 150AD work, you can see allot of east to west distortion but the north south seems to be reasonable to my eye.
Note that the map makes seas and rivers prominent but doesn't show land features like mountains. It is really a map of routes. Like a modern subway map, it is the line and the names that are important. The scale, less so.
Large sections of this map have been drawn from imagination or stories. Notice that the area east of Maysia is quite incorrect. An enlargement of this East Asian area is shown below.
Other Roman maps I have seen, such as this other map of the world, look odd to our eyes. Several conventions that we take for granted have not been followed. North is not at the top. The Mediterranean, the centre of the Roman world, is in the centre, and the area between Asia and Africa has been guessed and so no Americas. However, once you get past those issues, It does seem to be a usable depiction of much of Europe, Northern Africa and near Asia.
Many more Roman maps can be found by in google's image search.