"Size of a Elephant?"

I have a question for the savants on the list?
I'm looking at doing a FT campaign with my club, and I want to give one
side
(the invaders) a scary new weapon.
I rather like the idea of a meson type weapon, that has the possibility
a bypassing all armour.
This is the basic idea so far:-
Range 36mu ( about average I believe for most FT weapons)
Hits as Beam weapon
Damage
hit roll of 4 = 1pt (represents a near miss, 1off armour if ship has
armour)
hit roll of 5 = 1d3 (a glancing hit, 1 off armour, rest off hull)
hit roll of 6 = 1d6 (a hit in the interior of the ship, all off hull)
re-roll damage rolls of 6,representing secondary explosions 
Screens reduce to hit roll by 1 or 2 points

Now you are probably thinking, 'not very scary', but here is the kicker.
As I recall Meson guns in Traveller were affected by the configuration
and size of target
So the number of dice you roll is equal to TMF/30
For example if the target is a DD at 30 TMF or smaller you roll 1 die,
heavy cruiser such as a Vandenberg or Jerez (80 and 88TMF) 3 dice, BDNs
such as the Excalibur (140 TMF) 5 dice
and against a Jeanne D'Arc (280 TMF) 10 dice!!!

My query is, how big should this weapon be, to be balanced?

Tony