the normal size... you are not needing the magnum size, most men don't, they just like to think they do.
OK, enough with the size jokes... If you're circumcised, you should be sure NOT to use too big of a condom. It should fit snuggly over you, with no wrinkling or bagging, but should not be so tight that it feels tight. If you get a good fit, it should be like a second skin. I'm not quite sure how it's supposed to work if you're uncut. Be sure to leave a small space at the tip. Pinch the air out of a small part of the tip before you roll it down. If you wear one that is even a little too big, you have a good chance that it will FALL OFF while you are (ahem) having fun, or it could leak around the base. Not good. Not a paid endorsement, just personal experience... I like: Durex extra-sensative or Trojan ENZ. Standard Mighty_Thor advice: Go buy some, and experiment/practice alone, so that you're ready when that special moment arrives. Waste a whole box alone. If you want to try many different brands, there are 3-packs available at most quick-marts. P.S. The true gentleman always shows has good manners. "You wore it, you store it." Don't leave the used item laying around to smell-up her place. Take it with you, and dispose of it later.
Like they said - most condoms fit most men. That said, I've heard quite a few guys say that Durex condoms must be at least slightly on the "smaller" side, complaining that they don't fit right. I've only ever had one other male friend that complained about other brands being too small, and while we never had sex, he was quite 'well-endowed' and even he said that most of them weren't incredibly small/tight, but it was just a slight discomfort. As long as they're staying on/not slipping, don't seem overly tight or incredibly small and uncomfortable, etc. most condoms fit most men.
Thizzlemain - any regular sized condom with a "reservoir tip" to hold your sperm is good for a six incher. I don't think the fact you are uncut makes a difference here. You said that you can retract your foreskin, but it sneaks back over the head after you pull it back. Practice stretching it gently in a warm bath several times, and it may stay back longer. Check to see, however, if you have a short frenulum (the stringy thing in the upside down "V" under the glans). If so, that may be why your foreskin goes back up. The only problem you will have in intercourse is that on the outstroke, your foreskin may cover the most sensitive part of yor penis, the coronal ridge at the back of the glans, reducing your sensation a little. Cutted