I built some 4x8 ft grow beds, and used a 1 inch stand pipe with a 1 1/2 x 1 inch reducer on top. The bell was 3 inch pipe.
The drain & fill time was towards the 'longer' end of the range, but it seemed to work.

I had 2x1 reducers, but never tried them. They 'may have' sped up the drain time 'some'...'?' If I had used them, the bell would of needed to be 4 inch.

The 1 inch stand pipe, is towards the small size, for that size bed. I used it, so I didn't have to use
a bigger pump (= bigger electric bills... )


Don't forget about having a big enough sump or fish tank. Depending on your media, the GB will need about 100-120 gallons of water to fill up, before the siphon kicks in.