Please go through the indexes if you are preparing to have sex and have never bought a condom before. That little bit aside...
Magnum or Magnum XXL are usually necessary for girth, not length. Magnum is about 10cm longer than a regular condom, and Magnum XL is the same length as magnum but with even more width. A regular condom should probably be fine for you if you are only slightly longer and thinner. Try looking at places like
www.Condomania.com for insight.
Also if thinner, you might want to try polyurethane or polyisoprene. I haven't tried the polyisoprene, but my gf and I greatly prefer the polyurethane feel to latex. Polyurethane's issue is that it's not very stretchy for girth.
You should pursue the morning after pill *now* before you have sex, and keep one at your place and your girl's. It's effectiveness is based upon how soon you take it after an accident, so having it on hand is very important. She should only take one in case of an incident such as the condom slipping off, breaking, or the two of you forgetting one. Also, you should invest in spermicidal foam/film and apply it inside her vagina with an applicator prior to sex. Condom on you, spermicide in her.
Have fun