"That's about the extent of it" is a common expression in English. It means that you have summarized all the major points on a topic, or given all the information you have. So let's say someone asked you about your last vacation. You can tell then where you went, when you went, why you went, what you did, etc. When you have no more information to give, you would say "and that's about the extent of it". They would then know that you have nothing else to say/add. Hope that helps! ~ Jennifer

I normally hear the word "size" instead of "extent" but the meaning is the same. This is an expression you would use at the end of a description or explanation of something. It means: what I have said expresses the situation more or less fully.