I had always heard that it was about the size of a fist. So I did a web search and it's 8 to 10 centimeters long by 6 centimeters wide which would be about the size of a fist. I'm not sure how big it can get during pregnancy. Fibroid, which are common benign growths, can cause it to grow also.
The endometrium typically gets up to 16mm thick. However, if a woman doesn't have regular monthly periods, it can get thicker. And in the years leading up to menopause, it can get thicker too.
Are you concerned for some reason that you would like to share?