This is pretty awesome! I was thinking on making it possible to visualize the levels but I wasn't sure how to do it and FuSoYa had not much idea either. Good job!
Internally level size mode 0x1D, 0x1E and 0x1F have 0x38, 0x3B and 0x40 respectively, but the SMW level layout doesn't support that and it would require more 384 bytes of free RAM, yet you can activate it internally (but it won't work as you expect unless if you expand the level table RAM).
dw $0100 ; #$1D - max 0x38 screens [$380 x $010 = $3800 = optimal] [may glitch]
dw $00F0 ; #$1E - max 0x3B screens [$3B0 x $00F = $3750] [may glitch]
dw $00E0 ; #$1F - max 0x40 screens [$400 x $00E = $3800 = optimal] [max horz] [may glitch]
I'd like also to suggest you adding support for displaying subscreens (key F2), since they make it better to view the horizontal/vertical "screen" count. Also making it zoom as well since 1 pixel per block can be small on some monitors...
Plus use a select list instead of straight up/down counters, they're way faster to pick other modes (e.g. select tag with size=32).
Also maybe Layer 2 support? Probably I'm asking too much at this rate hahahah
Other than that, it's an excellent job from your part!