Unless you are very short any road bike of decent quality will have 700c wheels.

Gear ratios are a little bit of playing around, based on fitness, terrain, length or ride, etc. But a good place to start is in the 46/17 range.

A flip flop hub means you have the ability to use a gear on either side of the hub. Normally it happens one of two ways: a fixed cog on one side and a freewheel on the other. Or two fixed cogs on either side of different sizes. Seen much more rarely is a freewheel on each side of different sizes. No matter what the application is you should get whats called a fixed/fixed hub if possible. A freewheel can safely be put on a fixed side. A fixed cog cannot SAFELY be put on a freewheel side.