Good advice above on choice for goggles.
Regarding glasses comfort and the right shape/ head space are vital for your glasses but there are a few options to deal with that:
1) take your glasses when trying on goggles - be prepared for them to get caught in the goggle when removing, so be prepared for them being dragged accross your face when you take goggles off (you will easily develop a technique to reduce that)
2)the alternate is to take your goggles when glasses shopping (or borrow some goggles) try them on together
If you can then go for a spare set of paintball glasses. These will be a basic cheap frame that is quite small in the goggles.
3) try one of the online opticians that send you frames to try
Eg
http://www.glassesdirect.co.uk
I used their 7 day home trial, picked half cheap small ones to try with goggles and half good ones for normal life. I ended up just getting decent ones so the paintball idea was a fail, however I can see to shoot people, I may need to put a couple of balls down range and lead them in though
They do referral deals as well as the normal 2 for 1 etc, so if you fancy I can probably get a referral sent
4) a prescription goggle insert. These have been pricey in the past but I've found these guys (have not tried them though)
When you've picked a goggle use the template to try them for size
http://www.prescriptioninserts.com/guides/Insert-Template.pdf?utm_nooverride-1