Definition of size
Size is a niche term used mostly in live poker and especially in tournaments. What is meant by size is the money taken from the buy-in amount of a tournament. For example, if a tournament costs 100??? + 10????, the "+10???" represents the entry fee, rake or size. The size is usually 10% of the tournament buy-in amount.
Avoid tournaments with high rake
When you play tournaments professionally, you do it because in the long run you know you'll make money, you know you'll be profitable and your ROI will be in the green. But the size or the rake affects our ROI (Return On Investment) a lot. Most of the time you will pay a tournament rake of about 10% and less (if the tournament is a high buy-in, it may be smaller than 10%). But some tournaments will always have a fee of 15% or more. Avoid this type of structure because even if you are an excellent tournament player, a high rake will eat into your profits too significantly.