Step 1: Identify allowed coefficients.
Only even powers are allowed: \( i = 0, 2, 4, \ldots, 2020. \)
Step 2: Count even numbers from 0 to 2020.
Number of even integers = \( \frac{2020}{2} + 1 = 1011. \)
Hence, dimension of \( W = 1011. \)
Final Answer: \[ \boxed{1011} \]