I wanna create a game not with sprite-based levels, but with full-size prerendered screens (locations). Just like in point-and-click quests.
Is Construct 2 a good tool for this sort of task or game size (in terms of memory) would be too heavy to export for mobile platforms?
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I think you should be fine, as long as you keep some things in mind.
I can only guess but if you go for 720p png images you should be able to go for ~1mb per layout. Also depends on how many colors you use (how good it compresses) or you could instead use jpg or gif (lower quality, lower size). Or you could load the images from a server. (Requires internet connection though)