In LSL6: pic 100, 410, 570, 580 and more (so yes, currently template interpreter must support it)
In SQ5: pic 49, 56, 82, 100 and more.
They're actually fairly common, I guess typically used in scenes where there is no status bar. I doesn't appear to be supported in SCI1.0 (at least, I don't see any pics that big).
I don't think there's a "size" flag anywhere, it just depends on the size of the bitmap image included in the pic I guess. I never supported it for SCI Companion, because I wasn't sure of the specs.
The crappy thing is that I've hard-coded the size in so many places, when I really didn't need to!