Welcome to SRP!
A lot of this is covered in the Library - you definitely want to read through it and watch some of the videos.
The typical recommendation for a beginner is a 5/8 or 6/8 round point that is professionally honed (personally I'd recommend 5/8).
The smaller and larger blades require different angles and pressure than the 5/8 which all of the tutorials seemed to be aimed at. And you don't want to mess up the angle with a small blade or the pressure with a large blade - you will wind up with more than just a nick that a styptic pencil can handle. OK, you don't want to mess up pressure and angle with any razor, but my small blades are more sensitive to the angle and my large blades are more sensitive to pressure.

If you can find an antique store that has different sizes, you could hold them and see how you like the different weights.

Unfortunately, for you wallet, you will probably end up with multiple razors of multiple sizes within a year, so keep an eye on the classifieds and experiment.