cdiver2:
I must admit I get lost with this crop stuff. Im ok with 2x4 -10x12 always 2" larger on the long side but after 10x12 that rule seems to go out the window.
It's not all that complicated. "35mm" film has an active area of 24mm x 36mm. Reducing down to simplest numbers, that's 2H by 3W. If your print doesn't have that same height to width ratio, then you either need to crop or, less likely, distort the photo to fit. Just like in fractions, multiply both the 2 and the 3 by the same number to get a print size that is the same fraction or "aspect ratio".
The 24x30" is the same ratio as an 4x5 negative. That's more "squarish" than the 35mm format. It's also the same ratio as an 8x10", so if you've done that slide as an 8x10", just use that as the example for the photo lab.
The most common TV format is 4x3. That has been carried over into computers (800x600 pixel, or 1024x768pixel screens for example). That in turn led consumer digital cameras to be that same 4x3 format.
What brought all of this to my attention was simply that, very soon after getting a digital camera, I closely cropped some photos so they looked good on the computer, then had them printed at the local 1 hour shop. Surprise, surprise.... the 4x6" photos left out chunks as they put what should be a 4.5" x 6 " photo into a 4x6" space. BTW, if you look carefully at what comes out of the 1 hour shops, you will see that, in addition to cropping for aspect ratio, in order to ensure that the prints are borderless, they overzoom a few percent. On a 4x6" print the overzoom is about 1/8" on all sides, just enough to compensate for possible misalignment of the paper in the photoprinter.
On a small run of 4x6 prints, discovering aspect ratios and the resultant cropping was just a "oh, interesting." A surprise like that on a 24"x30" would be a bit more shocking.