The transfer port is critical, indeed one of the more critical areas in springer design. As example the Anshutz 335 would have been a full 1.5 ftlb more powerful if only they had not gone for the 5mm transfer port because they thought it might perform better.
As a rule most should be about 3mm or 1/8" but its a balance of compression ratio, stroke length and transfer port diameter which usually arrives at 2.8mm to 3.2mm dependant on the rifle. Its also good if they have quite short and in line transfer ports. The round the bend transfer port of the Webley pistols probably costs 0.5/ftlb as example.
The worst ever is that of the Webley Mk3. We might not have put a nicer, higher quality rifle together in terms of solid steel and sleek walnut stock but all ruined by a transfer port which is actually larger in dia than a .177 pellet. Even been known to lose pellets into the cylinder when firing up into trees. Robs 2ftlbs from the power and has a terrible firing cycle compared to a modern tuned break barrel.
The Airsporter was not subject to the same issue with a moderately good transfer port size, sat between the Mk3 and the ideal at 4.2mm but the long transfer through the loading port length always leaving it behind the BSA Mercury, despite the better barrel/scope alignment potential.
So golden rule would be don't have a port larger than the smallest pellet, or its going down into the cylinder at some point and will be bloody awful to shoot.
Sub 2.5mm starts to restrict flow incidentally..