I found a solution to match roll20's grid to maps with an existing grid embedded. I commented on a post below, but this bothered me so much, I thought I'd start another thread to see if this could help more people.
Link to original thread <a href="http://community.roll20.net/discussion/577/easy-way-to-match-grid-sizes#Item_26" rel="nofollow">http://community.roll20.net/discussion/577/easy-way-to-match-grid-sizes#Item_26</a>
This does not use the ALT button to size the map in roll20.net.
1) Download The Gimp and use it to open the map.
2) Crop the map including complete squares only, no partial squares. Try to be as precise as possible.
3) Count the grid to determine how many squares wide and tall the map is. (example: 25 width & 20 height)
4) Save the cropped map & import into roll20.
5) Change the roll20.net page size, to match the width and height of your map. (in this case 25 inches wide x 20 inches high, 1 inch=1 square
6) Drag your map to the top left corner, and stretch it down to the bottom right. If your crop was accurate enough it'll line up properly.
Hit me up with any questions, I've done this twice and no problems. I really hope this helps everyone, its been driving me crazy for a while.
Good luck!