The bike ideed looks like a 26" MTB. But it looks bigger than the 46cm you stated. Either way it sounds like it is too big for someone 5' 1" in height, especially a female. But some people like bikes sized on the bigger end of things. But don't get any 26" tire that says 26 x 1 3/8" on it. This is an oddball size and is not a true MTB 26" tire. You need a tire with a decimal in the size, eg., 26 x 2.1 etc.