There is a solution that I have used for a while which works well using java script. Just add a script to your bookmarks bar and type in the resolution size you want - so for my screen (1280x1024) I set it to 1280.
SO - copy and paste the following in a new safari window - and then save it to your bookmarks bar and call it "resize"
javascript:moveTo(0,0);window.resizeTo(1280,screen.availHeight-1)
if you have 1024x768 monitor size, then type 1024 in place of 1280 in the java script.
after you have your window set to the size you want while you are using safari, simply quit safari and it should remember as you last left it. sometimes it doesn't so what I do is make a new safari window, hit my "resize" javascript in the bookmarks bar, make a new tab, and then quit safari, then it remembers what size it was -- hope this helps..