So I've been trying to get back into animating recently but unfortunately ever time I try to create a new document and/or open a flash file, the program immediately freezes until I close it through task manager.

I've installed Flash CS6 to try it out and it works fine, but unfortunately I find the brush sizes way too limited.

Is there anyway I can fix the crashing problem with Flash 8 or add more brush sizes for CS6? Thanks for the help.

Unfortunately there isn't a way to add more brushes in CS6. That being said you can always use the zoom button to your advantage, there is ways of making the brush sizes not be hindrances.
As for Flash 8, Can I ask if you have the Basic Flash 8 or Professional 8?

Quote from Apex-Predator

As for Flash 8, Can I ask if you have the Basic Flash 8 or Professional 8?

I use Flash 8 proffesional, thanks as well

Quote from KyZ

I've installed Flash CS6 to try it out and it works fine, but unfortunately I find the brush sizes way too limited.

Quote from Apex-Predator

Unfortunately there isn't a way to add more brushes in CS6

Flash 8 can add custom brush size?

Quote from KyZ

I use Flash 8 proffesional, thanks as well

Maybe the size of the application is too big for your computer to handle. In this case you should try the Basic Flash 8.

Quote from guramecon

Flash 8 can add custom brush size?

They already have custom (preset) brushes in Flash 8.