Sure, thanks. There are two things my game needs to do which depends on size:
1. Procedurally generate tiles around the player - just enough to fill the screen (deleting old ones after the player moves away). To know how many tiles are needed to fill the screen, I need to know the size of the screen.
2. UI stuff, i.e. making the game's title larger (within limits).
I think I saw some anchor point and scaling stuff which seems relevant for point 2, which I've yet to try.
For point 1, I was first trying to obtain a size similar to that seen as system canvas size in the debugger. JS screen.width/height didn't work for that (because they included the debugger), layout size didn't change from the original either, and that's when I stumbled across PlatformInfo. I thought I'd experiment with using either the device or CSS size from PlatformInfo to see which one maps more closely to the problem at hand. I think it'll be CSS from what I've seen so far.
I'm not sure what the difference is between viewport size and PlatformInfo device/CSS size, so I'm honestly not sure if that's what I'm looking for? At the moment, I'm able to solve problem 1 and 2 using PlatformInfo CSS size. 2 can probably be done more conveniently using other engine features, but I'm pretty content with my solution for 1.