Pi-hole employs log rotation for its own log files, so file size consumption is limited to the amount of data logged within whatever period you've configured. If you stick with the defaults, Pi-hole rotates its logs once a day, at midnight, and keeps logs for the last five days.
Be aware that other components of your system employ logging as well, just as any Linux system would. This especially includes logs generated by lighttpd, the web server Pi-hole uses by default.
For a list of files and locations (including logs) that Pi-hole is using or at least aware of (if not controlling them), refer to