Typical general purpose sledge hammer weight?
Normal "full length" sledge is 8 lbs, normal "hand sledge" (short handle) is 4.

Excess weight may have two downsides - one is more wear and tear on you the operator, and the other is that you may get lower velocity on your hits - and since energy is mass times velocity squared, a hammer weighing 12 lbs still needs to make at least 81% of the velocity of the 8 lb hammer to get the same "hit" - so you have to swing 50% more weight only 20% slower.

Barring unusual circumstances, I'd suggest you stick with 8 lbs. It got standard because it works for most things.

....also, go find (and perhaps even patronize) a brick-and-mortar hardware store...while you still can.