you just need to preserve the ratio and you can enlarge it all you want.
if 1 inch equals one mile, and you double the size of the chart then 2 inches equal 1 mile. as long as you have the percentage that you increase the overal size and constrain the x and y it should be simple math. If you are using photoshop or paint, you should be able to increase size percentage wise.
Or to explain it differently.. If X (length of chart) and Y (width of chart) remain in the same ratio to each other, simply change the first number of the scale accordingly.. i.e:
original: 1" = 1 mile
now if you make it say 50% bigger you would multiply the 1" by 1.5 (1 X orig size + 50% original size) , the 1 mile remains constant. thus 1.5" = 1 mile
If you double the size it would be 1" x 2 (100%=1 +100%) So 2" = 1 mile
Same if you want to reduce the size. say you want to make it 3/4's as big, so 3/4 = 0.75.. so your scale would go 1" = 1 mile
thus for the new size 1" X 0.75 = 1 mile so 0.75" equals 1 mile
You could also work math on the second number and leave the first number alone.
The important thing to rememeber is that if you are only changing the size of the chart keeping the length and weight in the same proportion you will only need to change one number the other number must remain a constant.
if you have calipers you can cheat, using the original scale on the original chart, use the calipers and mark of 1 mile, now change the chart size on the copier or what ever your using, lay the resized copy out and remeasure your marks, that number now = 1 mile