Last but not least, I'd recommend looking into penis extenders. There's a whole category of them, in different sizes and skin tones and they are designed to do the job of extending what you already have, which might be exactly what you're looking for!
If I remember correctly, somebody here very recently talked about breaking out in a rash after sucking a Doc Johnson non-silicone toy, so just to be on the safe side, I'd stay away from anything that's rubber, PVC, real skin etc. - which basically just leaves silicone for a "soft" dildo.
I'd also recommend comunicating with your girlfried - she might be perfectly happy with the size of your bits exactly because they are not too big for her ;) and a big dildo might just feel uncomfortable... (Personally, I'm not considering buying a 5 " circumference - or bigger - dildo any time soon.)
Firstly, remember it's quality not alway quantity that matters and she probably doesn't care anyway. But there are penis extensions on LH & I haven't tried them myself. However, reading the reviews on them should give you good info, if that is what you want to try.
Just remember, sex is all about having fun! Good luck. 