The Entrance holes could be rectangular, square and some even made it round. All depended the on how the design of the farm especially the roving.
The sizes obviously related to the size of the roving and farm. Too big the sunlight can easily enter the farm and this is disliked by birds. Too small it is hard for birds to fly in. So you must choose a size that could accommodate both. But for new farm a bigger entrance hole is much better than a small one.
(entrance from 2 sides)
There are no best or standardize entrance holes it all depended on the farm's design as whole.
Other consideration would be the entrance hole position, facing the East, West, etc. All these would affect or justify the appropriate entrance hole sizes.