what is the best size bird cage to use until i upgrade to a bigger one?

Save yourself a huge headache and extra expenses, get the biggest cage you can afford, make sure the bar spacing is no bigger than a 1/2 inch apart.

Next, get a glider safe wheel, You will have to order it online. All the wheels you will find at pet stores are not designed or safe for gliders. Wheels designed for gliders are a bit more expensive but they don't injure gliders and they last a long time.

Then pick out a nice fleece set or two for the cage and plenty of toys and a bonding bag.

We made the mistake of not doing any of this and so far have invested

small unsafe wheel 15
large unsafe wheel 20ish
Safe Raptor wheel 55
small cage 60
larger cheaper cage 45
Larger sturdy cage 130

Not to mention the fleece sets and bonding bags and toys and food etc....

Thats roughly 140.00 that we didn't need to spend on stuff we used for maybe a month at the most. I think we actually spent more than that but this gives you an idea...

Get the cage you plan on keeping them in long term and maybe a small cage for travel or to use in an emergency for a hospital cage. The cage needs to big enough for two gliders, even if you get joeys they will appreciate the room. Small cages are just bad investments unless you are buying them for travel purposes. Gliders get bored easily and will start doing stress circles if the cage is to small, with nothing in them to stimulate the glider they neeeed toys and things to interact with.

This is the cage I have my pair in, its comfortable for 2 maybe 3 gliders and we are actually in the process of putting two of these together to increase their living space
http://www.exoticnutrition.com/brsuglca.html