There are two factors to consider here when using Tower Buff Beams.
Length: The length of a Tower Buff Beam has no effect on the stats of a beam. A super-short beam has the exact same stats as a maxed out length beam. The only difference here is the amount of defense units it costs to summon it.
Multiplier: The multiplier of the buff beam is dependent on your Series EV's tower stats. The higher the stats, the higher the multiplier. These multipliers scale differently; damage scales the best, followed by damage resistance, attack rate, and lastly range (range scales horribly).
It is not uncommon for a seasoned player to have two Series EV heroes; one geared specifically for tower damage, and the other geared towards tower health. The damage buff beams would be used only on damaging towers like harpoons, while the tower health EV only summons physical beams and buff beams to make them even more resilient.