Thank you both,
for a short time I can use a S7 edge and I'm surprised how different per example my theme looks to my Note 2 with the S7.
So, it seems to be necessary to have themes with different resolutions as already offered by Tobias at Openandromaps.
I can surely try to use an emulator, but I fear it isn't the same as to test it with a real smartphone, as John wrote.
I found a solution for it with a script, that can increase the symbols, stroke-width and letter size and using MTP copy a short test with different smartphones doesn't take much time. But because a S7 is very expensive, I'm looking for a cheaper solution.