There was not any constant size for their navy. Their ship building was institutionalized. Ships were built for different purposes, such as war ships (like kanni, mandalam, thalam and a few more), food supply ships and trade ships. It is widely said that the size of the smallest fleet would have contained about 12 ships and the number grew larger based on the purpose. As estimated in wikipedia, they might have had up to 1000 ships during their peak. There is no estimate for man-power.
The first attack over the Sri Lankan land was against
Mahinda V. It was held around 981 CE. Not as much as 500 ships were used for this attack. Still the exact count is not known. There was an internal military rebellion happening in Mahinda's land by that time and that made it considerably more easy for the fleets of
Raja Raja Cholan I to intrude into Sri Lanka.
The attack was led by
Rajendra Cholan I, son of Raja Raja Cholan I. His attack is considered to be one of the greatest naval attacks in the history. Even the name of the Indian National Naval Academy from 1972 to 1993 is
T.S. Rajendra Training Ship Rajendra.