I am currently working on a quote that requires me to add a step down transformer (480vac/208vac). I mostly deal with Low Voltage/Dc Power and do not have a clue about sizing the transformer. I am looking for someone who can help me size it correctly. I know that the source/line voltage is 480vac, the load current will be (2) 200amp feeds going to 208vac inverters. Also need to know the fuse size for feeding the transformer. I appreciate any help you can provide.

Normally you size a transformer based on secondary current requirements. In your case you have indicated (2) 200a loads at 208v. The following formula applies;

KVA = IxEx1.73/1000 where I=current, E=voltage

400*208*1.73/1000=143.936 KVA Transformers are available from many suppliers and you will find a 150KVA is a standard size for most.

Fuse primary side at 225a

(150*1000)/(1.73*480)=180.6*1.25=225