Should I crop to a specific size or custom to the subject? Does it matter how I crop if I want to print different sizes. Ultimately I want to crop my photo's and upload them to smugmug so family/friends can order prints.

I am still a noob but I have a picture of a person but there is a lot of extra stuff in the frame that I want to remove. Should I crop it as a 4x6 or 5x7 or some other custom size? Maybe I am missing something here...

then print whatever size you want that's able to fit for that picture.

it probably has some printing presets.

Generally you would first crop the image to your liking and then adjust the resolution (usually 300 or 240 DPI for desktop inkjets) to match the desired print size.

With printing services you have more freedom as they can uprez images better...

Some applications offer cropping already preset to standard print sizes - don't know if Aperture has it.

1. You have the desired elements in the frame
2. It will handle whatever variety of print sizes people will order and still look (or close to) the way you want it to.

For example, I do most of my motorcycle pics in 8x10 ratio, giving a little extra room top and bottom (or left right if in portrait orientation). That way, if the purchaser wants a 4x6 or 5x7 ratio, it's the top and bottom extra that get cut off, not anything important.

<img src="http://tristansphotography.smugmug.com/photos/136804284-M.jpg">

Just leave a little extra on the sides if you think people might want 5x7 or 8x10 copies.

with the constrain option checked. That is aspect ratio for 4x6s which is what most people consider a print. If you're family is like mine, thats what they'll order 95% of the time.

When you're ready to print click on the image, and hit SHIFT + APPLE + E. Thats Export Version. One of the options is JPEG - 4x6 prints. Save that one for family prints.