Put your brain into gear. If you have a 128x64 OLED there are practical difficulties in fitting "big" digits on the screen.
e.g. 1 line with 4 digits that are 32x64
e.g. 2 lines with 8 digits that are 16x32
Bear in mind that the lit pixels will be smaller. You have to allow a space between each digit.
You can convert a TTF font on your PC for U8g2 format.
Or convert most other font formats to Adafruit FreeFont format.
It can be a bit fiddly to do the conversion. But if you can show the exact font with exact size that you want to use, someone might do it for you.
If you want a "normal" font with letters 32-127 there are online tools.
For restricted fonts like digits-only, you need to use command line tools. (Or someone might do it for you)
In practice, I would choose the "most attractive" font that comes with your library e.g. U8g2 or Adafruit_GFX.
If none are suitable, be clear about exactly what "custom" font you want.