Bruce array is a frequency scanning array antenna, which means the array radiation pattern is changing with scanned frequency. The array should scan the beam vs frequency from backward endfire (min frequency), through broadside (mid frequency), to forward endfire (max frequency).
Theoretically the horizontal and vertical segments should be λ/4 at the center of the frequency range.
If you need only broadside pattern (center frequency) you may optimize the element lengths for maximum broadside gain (usually the horizontal segments first).