First of all it's worth asking: is this really a problem? Modern devices all have loads of memory, and it doesn't look like your project is that image heavy. So long as the peak memory usage is reasonable, then optimizing it is actually a waste of time - you may as well spend your time on something more significant.
So is your peak memory usage actually a problem? If so the first place to look is our guide on memory usage which includes general advice on optimizing it.
If you really want to adjust Construct's spritesheets, there are two things you can do: the main one is to adjust the project advanced setting Max spritesheet size. Lower maximum spritesheet sizes will allow finer memory loading granularity which can result in lower peak memory usage, but at the expense of efficiency (reduced runtime performance and increased download size).
The second thing is to make sure you've organized all your objects on to layouts appropriately. Construct has an algorithm that tries to group objects used together on the same layout on to the same spritesheet, in order to avoid situations like having one spritesheet with content from several different layouts that are never used together. If your project doesn't abide by that (e.g. by placing everything on the same layout) then Construct doesn't have any clues about how to group objects together and so that algorithm will prove ineffective.
I'm skeptical that any feature to manually adjust spritesheets would really be useful in practice - with its current design, Construct frequently discards and regenerates spritesheets as you edit your project. So any manual changes would likely be quickly reverted. Even if there were other ways to permanently specify how spritesheets were arranged, as you continued to edit the rest of the project and Construct continued to automatically regenerate other content, I'd imagine it would be easy to accidentally end up with a set of spritesheets that are actually less efficient than if you just left Construct to do it automatically - and that might be after having to spend a long time to manually tweak everything. It doesn't seem like a good thing to spend time on when designing your project, especially if the peak memory usage is not really a problem to begin with.