To find the length of the base of the triangle, we can use the formula for the area of a triangle:
\[ \text{Area} = \frac{1}{2} \times \text{base} \times \text{height} \]
We know the area is 7.5 square feet and the height is 2.5 feet. We can plug these values into the formula and solve for the base.
\[ 7.5 = \frac{1}{2} \times \text{base} \times 2.5 \]
Now, multiply both sides of the equation by 2 to eliminate the fraction:
\[ 15 = \text{base} \times 2.5 \]
Next, divide both sides by 2.5 to solve for the base:
\[ \text{base} = \frac{15}{2.5} = 6 \]
Thus, the length of the base of Ja'Miles' sign is 6 feet.