I haven't tried this yet, as I don't have a card reader, but seems harmless enough. I found this simple tutorial on another site. Just remember not to place them in the phones memory, but on your memory card. I'm also fairly sure that the size of fonts can't be changed but you can change the type which may help.
Here is how:
You have to have a card reader. On the card you have to create a folder FONTS in the resource folder and paste contents of one of the zip file to FONTS, put the card in the phone and restart. The fonts should be used. I have to warn you that installing fonts on the E: drive (which is the expansion memory) will turn off the ability for the phone to in the Data Transfer mode, this is due to the fact that the phone has exclusive rights to the font files after you place them on the memory card. You should never place fonts on the phone's memory because then you would have to format the phone to get rid of them. When you want to change fonts you will have to replace the ones in the E:\resource\fonts with a new set of font files. You can only do this with a card reader, the phone will not let you rename, delete or copy-over the font files once it starts using them.