When you're starting out, a 5/8 or 6/8, round tip, half-hollow is a pretty good neutral ground to start with IMO. Very small difference between using a 5/8 and a 6/8 other than preference, if you're using 5/8 as a standard.

As far as aggressiveness, once you're comfortable with shaving with a straight, it all comes down to it's only the edge that actually shaves your face, not the blade, and size then comes down to preference with different attributes to each size, but you'd probably different levels of aggressiveness more in the grind than in the size IMO.

Once you understand you're shave, experimenting is all the fun. I used to see video's of members here using huge blades and would laugh, thinking I would never use a blade that large, now they're my favorite. Also have a pair of 4/8's that are quite awesome blades, and equally like those.

I'm not really interested anymore in a blade smaller than a 7/8, with the 9/8's being my favorite! Sheesh, when I use a 5/8 now, it almost feels like using a toy, yet when I started out, a 5/8 felt like taking a cleaver to my face.

Your preferences will all develop as you go....and that's a lot of the fun!

Cheers!