Reluctant Condom

I'm 28 years old and still a virgin. I'm also uncircumsized. I'm not sure of how big my penis is because for a while now i've been dealing with erection problems due to high blood pressure and stress. i'm am currently remedying those issues. Anyway, a while back, on a good day, i was luckily able to get a fairly decent erection and decided to try on a condom for the first time, so that i would know how to when that special moment comes. the condom was a popular brand and was standard sized. I rolled back my foreskin and tried the condom on as was directed. It felt too small, i had trouble getting it over the head and struggled to get it over the shaft. It didn't quite reach the bottom of my shaft either. Once on, it felt a little tight but not too uncomfortable. Do i need another sized condom, a special size maybe? I'de hate to buy magnums if they're not necessary. They're more expensive and can cause an embarassing moment if they're too big. I'm not sure of how a condom should fit and can use some advice. Thanks guys.

By the way, i love this site. It's awesome!