You can't really go smaller with a Riva. The smallest Riva battery is 650mah and I doubt they are much smaller than 750mah.
Or you do mean skinnier? NOTE - do not buy auto
batteries. Manuals are what most people here use and recommend for a variety of reasons. A pink manual mega xl Joye 510 would be 280ma, skinny and about 128mm long w/ a cartomizer. It would look slick - pink battery and black carto. But you'll have to see if you can find a pink mega kit. Not a good price here but this is what it would look like (but shorter with a Boge carto instead of atomizer and cart). The PCC you can find elsewhere is non-roomy metal:
Joye 510 Mega Kit (Pink)
The standard size manual 510s would be another option, with a PCC (NoSmokeVA.com has a nice PCC for the standard) but the standards are only 180mah and that is about 1.8 hours of moderate vaping.
OR head over to BoxMods.com and check out the Bic-lighter-sized "Little Sister 3.7v" so so cute and can be ordered in pink. That's a mod and it uses removable industrial type batteries. I am sort of drooling over that... (though not actually sure if it is smaller than 750mah Riva)
And for future reference. Slim 510's cannot use LR atomizers, LR cartomizers or the fast type of Riva/Ego charger cable. Any of those can damage slim 510 batteries.