How does one determine what size bow to buy? I have noticed many lengths and draw distances. I don't want to spend a lot of money on a bow to find out it is not comfortable to use.

The best way to buy a bow is to go to a shop where you can try them out first. This will allow them to measure your draw length and decide on the right sized bow for you, and for you to find out what bow you are comfortable with. It depends on your arm length and height really, but it's still best to try them. Most good stores will help set the bow up and allow you to bring the bow back in original condition if you still don't get on with it after a while.

I'm in the UK, so the shops I use are Quicks and Wales Archery are supposed to be good.

Saker is right. You just have to try the bow to get the best bow for you. I still shoot some of the older longer frame compound bows, but the shorter frame bows are much easier to use in a tree stand, and they are lighter than the older ones. You just have to try them to find what you need.