Are you trying to do a pot melt into a shaped mould? If so you will have a shaped bottom and a flat top with no depression. If you mean to melt to a square and then slump it, that is a different proposition.
But answer the question put in the first sentence. Measure in centimetres – the calculatons are much easier. the thickness is 0.6cm, if you want a square 30cm square you multiply length time width times thickness. 30 x 30 x 0.6 = 540cc. Multiply this by the specific gravity of glass (2.5) = 1,350 grams of glass = 1.35 kg = almost 3 lbs.
If on the other hand you mean to have 6mm thick walls on the drop, you are asking a completely different question. A 75mm drop will require less glass than a 200mm drop. I would guess – and it is complketely guess work – you would need 12-15mm slab to start with. For 200mm, maybe 25 -30mm slab would do it. The problem is the sides. They thin dramatically as the glass heats up. So the thicker the glass bottom the faster the glass will fall and stretch thinner.
Seems to me to be better, if you really want to do this, to emulate the Norman slab process. In this the bubble is started and then blown into a tall square steel mould.