I am looking to buy a used beach cruiser online, I am 5'1" what size bike frame (in inches) would work for me?

I have been looking at charts that say 20" but those are all childrens bikes! Would a 26" bike be too big?

The 20" and 26" figures you've seen on the charts might be referring to the wheel size. Different frames from different manufacturers might vary in size even though they say they measure the same. Since you're not sure what you need, you'd be better off going to your local bike shop and getting properly fitted and take the bike for a test ride.

Buying a cruiser on line might sound like a good deal, but since they're so heavy, the shipping might cost a lot. And if it needs some assembly beyond your techincal abilities, you'd have to bring it to the bike shop anyway where they would charge $50-100. Plus if you establish a good relationship with the bike shop, you'll have a place to go for maintenance & repairs.

Yes, 20" wheels are too small for a cruiser for you, 26" wheels is the norm. The problem is these bikes don't really come in frames sizes. One bike will simply fit you better than another. At your short height, I wouldn't buy on-line as you really what a smaller model and you can't try it before you but it. Try Craig's list for a used one. Yard sales and some bike shops may have used as well.

26" is about the wheels..... 26 inches are normal bikes wheel size.. all mountainbikes are 26"....

your FRAME size would be around 14" ....but it's hard to say in this case... but if someone said that cruisers come without frame sizes then theres no way to get the right size bike for you on the net. and yes, some bike shops sell used ones as well. although I don't see a problem to get a new one- a lot them come for not more than 400$, even cheaper