A medieval busy Tavern, the stereotypical tavern - the staple of the standard D&D game. A busy common room, with different races and classes all doing what they do. The wizards studing their tomes, perhaps a flash of arcane magic. The theives lightening pockets, perhaps an ace or two up his sleeve during a game of cards. The warriors quality testing the taverns ale, wearing their armor and weapons at the ready just in case.

Id also like to incorporate the likeness of real people into it as well. As well the more you look at it the more you see. If that make sense,

Size - I don't know - large enough show detail but not mural sized
Cost - never commissioned anything before