Is it normal for dominant arm bicep and triceps to be slightly large than my left arm. muscle imbalance? no weakness. neuro for emg to check als?

Yes: I wouldn't call it "normal." It is, however, practically universal.

Dominant arm strongr: Since ALS is a painless weakness, and you do not have weakness, ALS should not be considered the cause. If you use the dominant arm for weights and do not use the other arm, the muscles will be different sizes. Normally they are pretty close. The easiest way to sort this out is to see if you can make the left arm stronger. That would rule out an illness.

Not uncommon: OK to have some imbalance and asymmetry, and is NOT a sign of ALS, unless you have progressive weakness, muscular atrophy, and fasciculations all together.