Ford 2150 2 bbl

The venturi size should be in a circle on the side of the carb, possibly hidden by linkages or brackets. There's a photo of where to look here. Info on available sizes is here (scroll down; not all the clickable FAQs are actually clickable).

If the number is obscured, blasted off by past abusive cleaning in a "remanufacturing" operation, or otherwise illegible, do you have a Ford number stamped or tagged on the carb you intend to use?