1
general::
That sums up the situation; that's how things are. For example, So he's going to resign next month?? Yes, that's about the size of it, or Mary's applying to all those colleges?? That's about the size of it. A mid-19th century British expression that soon crossed the Atlantic, it appeared in Mark Twain's Tramp Abroad (1880): "‘Bloodshed!' ‘That's about the size of it,' I said."
American Heritage Idioms
2
general::
Phrase(s): That’s about the size of it.
Inf. That is the way it is. (Often a response to someone who has acknowledged bad news.) • Bob: We only have a few hundred dollars left in the bank. Sally: That means that there isn’t enough money for us to go on vacation? Bob: That’s about the size of it. • Bob: I’m supposed to take this bill to the county clerk’s office and pay them four hundred dollars? Sally: That’s about the size of it.
McGrawhill's American Idioms And Phrasal Verbs