If you're on console/switch it should tell you in the tooltip when it's selected in inventory. No limit on PC, of course, so I don't know for sure of the top of my head.
The weighting is not a simple volumetric messurement, it takes into account how much CPU/graphics burden the block could take up. So complex and crafting blocks are disproportionately higher. And each separate tech takes 200 allowance, even if it's a single anchored block. Sorry, I know you din't ask any of that. Just covering all bases.