I use elementary on a 13inch laptop with FullHD resolution. But much of the system is just too small for me. The size of the system's Font I have already made greater with "accessibility" in the settings. Unfortunately the control elements (close / full picture) are still very small. The same applies to the icons in the system control and much else. Is there an easy way to change this? (I don't like the solution to reduce the resolution)
One other option would be to tell elementary to render everything at 2x, then scale your resolution down to 1.5x, as described here. This isn't an ideal solution - you'll still get some blurriness, and graphics performance may be a little slower - but it would solve the problem, and it would look better than just reducing the resolution.
There isn't an easy way to set up true 1.5x scaling on elementary. As you mentioned, changing the font size to "larger" helps a little (and will make icons a bit bigger), and changing the resolution would fix the problem at the expense of making things really blurry.
This post from elementary's UX architect explains why elementary provides 2x scaling but not 1.5x scaling.