I want to buy a banana seat bicycle...?

Question

but am unsure if a 20" bicycle is too small for an adult.

I am 5'3 and average build.

the bikes i've looked at are on bicycledesigner.com.

any help?

Answer 1

The bikes your looking at are low riders. They aren't made for comfortable riding. With the seats so low they would be hard to ride for you.  

Answer 2

Any bike that allows you to extend your legs straight while your peddling is best for the exercise value. I remember having a lowrider (Dragstar) as a kid.