Definitely shoot in the highest resolution that you can manage. You can always downsize an image, but never upsize without horrible loss of quality.
You might want to crop, or you might want larger images in the future when new technolgies come along, or you might want to print a picture even if you don't realise it at the time of shooting. It can be better for some post-processing as well. Keep your options open.
Maybe you won't think all that is important, but cropping in particular is often immensely useful, epecially when you're not one of the best photographers with the best equipment. So shoot high resolution even if you don't care about all the other points.
To make a smaller copy of a picture, you can use a simple free program like Irfanview, if not getting more comprehensive image editing software.
