ZINFGAARON: check some online tire size calculators. For example like
this. There you can see differences of diameter/circumference/speedo reading difference/ride height change and such. Or made for our cars that also show wheel clearance
this (change tire placement for both to be front or rear, if comparing tire size fitment, not same tire set on front-rear).
Simplest bet if you don't wish to change much resulting performance is to keep at least circumference somewhat similar to that of stock tire size's. After all, different circumference = different gearing ratio. So noticeably bigger diameter/circumference tire will make gears "taller" (longer distance travelled for same count of wheel/engine evolutions), resulting in worse acceleration (but higher top speed, which most probably you almost never need of unlike acceleration).
To get that usually means when using bigger diameter wheels (R17>R18), reducing at same time height of tire sidewall (profile, which is given as % from tire width), and vice versa, if you install R16 wheels, worth proportionally rising sidewall height. Note though, that as profile number is % of width, not specific length measurement, it changes also for different widths. For example, if you want to install narrower winter tires of 205 width vs 225 summers, then it's worth to up profile/sidewall a step, from 225/35/R18 to 205/40/R18.