I am planning to convert my garage into a bedroom and the location of the existing panel is where I would like a storage room / walk in closet to be. My question is how big does a room have to be to no longer be considered a closet? Could a storage room / closet combination still contain a panel that maintains the 30 inch access restriction in front of the panel? In this case the room would be approximately 7' X 10'.

No. A clothes closet is still a clothes closet, no matter how big.

I suppose you could build a panel closet inside the w-i closet, as long as the proper clearances are met inside the panel closet, but IMO this would not be an efficient use of space.

Thank you for your input. I was afraid that it would boil down to this. I hadn't considered a panel closet... how big would that have to be... 3' X 3'? If it had it's own door and wasn't used for anything else would that be okay?

The inside dimensions would have to be MINIMUM of 30" wide (side to side, with the panel anywhere in that 30" space) and 36" deep (in front of the panel).

Although, this may still not fly with your local inspector. Best bet is to check first.