Since the drawings are similar, the lengths of the 2 sides are proportional.
Since the largest size is 10.5 for the new drawing, it corresponds to the large side of the former drawing, which is 5.
Similary, the corresponding side of 3 (in the former drawing) in the larger drawing is what we are seeking to find, let's call this x.
So the ratio of 10.5 is to 5 should be equal to the ratio of x is to 3. Setting up the ratio, we have:

Now cross multiply and solve for x, we get:
.
So the length of the side we are looking for in the former drawing is 6.3 inches.
ANSWER: 6.3 in