Size doesn't matter. (h0h0h0)
PHP is server-side, so clearly it's not what you'll be using.
As described above, you could use CSS, but that's not very good. Fixed font sizes tend to suck.
Take for example MSN.com. It's got that little microscopic font for everything. No one with normal humans should be able to read that with ease.
Thankfully, you can resize it.
Anyone in their right mind will consider fixed font sizes very unprofessional, so I strongly advise you change your tactic.