I think u must be planning to draw a square for solving some excercise. You can draw any square and title side as any length. e.g draw a square of 6 in side and title side as 24 in. and add footnote as "scale 3:1"
#1 option:
8.5 x 8.5 IF you are limited to using only the 8.5 x 11 sheet of paper AS IT EXISTS (i.e., without being scaled, as in Option #2)
#2 option: use a scale, as suggested.
For example, you can make an 8.5 x 8.5 square and add a "Legend" that explains,.for example, that one inch = 50 miles, in which case the scale on the 'legend' would be 1 = 3,168,000; you could make that 100,000 miles, if you wanted...in essence, ANY measurement, as long as the scale in your legend is posted and is correct.
Sorry, yoboy, but a scale of 6 inch = 24 inch is 1:4,
NOT 3:1