What size should they be between? in mm please. thank you.

In flights, about 12mm... and up to log sized.

In cabinets i like to use one 16mm and one of the next size up (dont remember what size that is)

Show cages are 12mm soon to be changed to 16mm

I need them for cabinents. I can get them in these sizes. 9,12,19mm any other size I would have to go to another town. Which would be better the 19 or 12 I be in another town tomorrow where I may be able to get 16.

19mm will be best. 12mm is quite narrow and many birds cant grip it properly to mate.

Try using 19mm square, with the sharp edge taken off with a planer. Enables hens to grip the perch better when mating.

Some of my breeding cages have square perches and the hens do seem to grip better

My breeding cages have two sizes of perch...One is the 19mm square with edges rounded off with a plane like Heathrow suggested and the other perch is a 16 mm round. I rarely ever use anything under 16 mm for show sized birds.

Mine are the same as Kaz's