Condoms vary, of course, by brand name, but across the board, condoms slightly differ in their measurements -- and by measurement I refer to your length and girth, but also the thickness of the condom itself.
Before we begin, it's rather important that you, the condom purchaser, take a proper measurement of your erect penis. Most measuring sites will tell you that to properly measure, you must...
1. Work your way to a full erection.
2. Pull down your erection as far as parallel to the floor as possible to get an accurate measurement.
3. Perform a "bone pressed" measurement, meaning that you line your ruler's 0 marking fits at the base of your cock, hold the ruler firmly against your erection, and read the measurement at the tip of your dick.
4. For girth, secure a tailor's measuring spool and wrap it around at midshaft. Wind the spool around your dick and note where the metal end overlaps for the measurement. For fairness, compare that midshaft measurement with the girth at your thickest part on the shaft. And if you don't have the spool, simply tear off a strip of paper and repeat the process -- using a ruler to measure how long the paper was at its overlap point.
5. Of course, you can always take a simple measurement of width (across the top of your dick) since the condom dimensions below are length and width (not girth -- to get girth, multiply by pi).
Simply put, you're cheating yourself on a good condom fit if you hype up your measurements.
There are three incredibly general condom size classes: small, medium, large.
Snug-sized and average-sized condoms, across the board, best fits a five- to six-inch-long penis with an inch and a half to inch and three-quarters width. These "regular" sized condoms are frequently displayed on shelves at your local grocer or discount store.
The condom brands featured on [link=http://www.ilovexor.com/101condoms.cfm]this particular directory[/link] average about seven by two (a bit larger than standard issue) and feature such popular brands as Trojan and Avanti. Again, unless it specifically says "large," condoms usually fit in this broad range (5-7 x 1.5-2). Quite a few condom brands list accommodating measures up to eight inches.
Larger condoms like the Magnum, the Magnum XL, and the Lifestyles Large best fit sizes 8" or more in length and 2.5" or more in width (5.5-6 girth at estimate, more with the XL brand). Trojan's own advertising asserts that the Magnum and Magnum XL brands are 15% and 30% longer respectively than their average line, and are significantly larger in girth.
It's important to find a good fit for a condom. Condoms that are too tight can easily break during sexual intercourse, and needless to say, will make your johnson feel too constricted. From personal experience, a too-tight condom diminishes my erection, simply because it's too painful and tight to be comfortable. Condoms that are too big can easily slip off.
Your best bet is to do some condom experimentation. Since you can pick up free condoms at most clinics, hospitals, and your local student nursing facility, it couldn't hurt to grab a couple and see if the brand's right. Condoms are relatively inexpensive and easily available; for larger brands, it doesn't hurt to shop online. [link=http://www.trojancondom.com]Trojancondom.com[/link] and [link=http://www.condomania.com]Condomania.com[/link] are good places to start. You might also want to embark on a Google search for [link=http://www.google.com/search?sourceid=navclient&q=condom+length+and+width]condom size dimensions[/link] to do your own comparisons.
In any case, wrap it up every time. Sex is too risky nowadays to do without protection!