Changing the default font size and family can make your fingerprint unique, so it's strongly discouraged.
Many sites use JavaScript based font probing to fingerprint users by printing invisible strings with different fonts and then comparing the dimensions to that of fallback fonts [1, 22, 3].
Tor Browser limits the number of fonts a page can use, but it doesn't spoof the dimensions of the printed strings.
So, a more advanced script may report the dimensions of the strings printed with different fonts. That would uniquely identify your browser, even the number of fonts available to the script is limited. Admittedly, that wouldn't be the most effective fingerprinting method, but I wouldn't bet my anonymity on this.