PSA: When using sizables to make decoration or custom tiles, please take note that the sizable limit is 1000. Any more of that and SMBX will crash upon starting the level!
Since PGE's default counters do not have a counter for sizables, you can go to Debugger, and create a custom counter that covers every single basegame sizable, so you can track down how many sizables you have in your level. Keep track of this if you do plan on making custom blocks with sizables (and 8x8 gridsize too).
1. Go to Debugger
2. Click the Add button under Custom Counters
3. Item type is Block, and you can set a name (for convenience, name it "Sizables (Max 1000)")
4. Click Add and then choose Sizable Blocks category (with group set to "All"), then click and drag to select all of the basegame sizables, then press OK. Then press OK on the previous window to add the custom counter.