The blocks I put in were about 12" x 4" - I figured that was enough surface area to distribute the load - or at least to take on some of the load.
The shower appeared to be held in place simply by 3/4" wood screws around the opening. The enclosure moved freely once I removed those screws. Due to the shower flexing / shifting, many of the screws had been pulled out of their original holes in the walls. I could not see any other support, anchor points or blocking anywhere.
Once the new blocking was in I filled the original holes with polyester compound and then re-drilled and screwed after it hardened - my thinking was that this should provide additional support.
I was surprised how large the cavity was and how much wiring was underneath the shower pan - both 12V
and 120VAC Romex under there.. While it's technically a dry area, I was surprised the design doesn't call for more protection for the line voltage wiring.