Check out this thread:
The chart will give you an idea of what gravity you can brew based on your efficiency and mash tun size. If you're just starting all grain, I'd plan for about 70% efficiency when looking at the chart so you don't under size the tun while you learn. As you improve your efficiency you'll just have a bit of extra space.
Unless you're doing a BIAB process (which has its own sizing needs). Then you'll need a separate boil kettle. 15 gallons would be a good size for 10 gallon batches (converted kegs work great for this).