[MrForbes]

Some idlers are fixed, others are spring loaded. An example of a fixed idler is the one on the supercharger drive on the car in my avatar.

The short answer to the original question is, it does not matter what size the idler pulley is, or where you put it. It might be best to put it on the slack side of the belt, but any location on either side will probably work ok. The idler could push the belt in, or pull it out. The idler pulley radius needs to be larger than the minimum bend radius of the belt, that's all.