Hi Jas
I suggest you have a read through the stellafane site on mirror making to learn the basics. Here is their section on determining the focal length:
http://stellafane.org/tm/atm/grind/measure.html
You can have any focal length you like with any diameter mirror. In practice most mirror makers with 6 to 8 inch mirrors got for a focal length 6 to 8 times the mirror diameter. That gives a good general purpose telescope and is relatively easy to make.
Rod