According to this post from Apple's Safari Support discussion forum:
1) Create a file named defaultzoom.css (or any name you like, just make sure it has a css extension.)
2) Copy and paste in the following:
change 130 to whatever number suits you.
>100 means zoom in,
<100 means zoom out. Don't forget the % percent sign!
3) In safari, go to
Preferences > Advanced. Under style sheet, select Other... and point to the file you created.
4) You may need to restart safari for the change to take effect.
[In the above, both the emphasis and code formatting (for non-code-related text) were added to the original quote by Am1rr3zA].