Never had a problem with the Stihl saws when i used to use them long ago, they were good for at least a 24" bar. Bigger than that was an old Husqvarna 36" never had the need to go bigger. Bar length depends on what you want to cut, but would have thought 18" would be more than plenty, don't forget gloves, hat/face mask, trousers and steelies as a minimum, for me i always wear jackets also after seeing and having a couple of near misses. Other than that also get yourself a nice axe, can't beat an hour or two chopping and stacking logs for a bit of exercise.
One way to try out sizes is to get down to your local power tools rental place, hire one for a day and see if it is up to the task.
Dont forget to sharpen your teeth every few times to keep them cutting nicely rather than burning through the wood, the saw should do the cutting with a minimal of force from you, you're just there to guide it and stop it jumping out!