What size of screw do I need to fix a 26" TV to a brick wall?
Use the longest screw you can without excessively damaging the wall. It's far better to have a more stable installation than chancing the television from falling.

Looking at it another way, if you use screws that are too small and are forced to redo the job, you're going to have more damage from the original installation than you would have if you had just used larger screws to begin with.