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If your Webers are optimised your Max power will be similar - but the torque, the shape of the whole curve, you will not believe the difference. The minimum size, well all the air in your DCOE needs to go through a Venturi of X mm. Go any smaller you will loose power. If you have a really big TB you will loose gas speed at the bottom end, costing torque (still better than Webers, though). Not knowing your power potential I can see no reason to go bigger than 45mm, and if you can find something cheaper that is bigger than the Venturi start with that. I had a Suzuki GTi that I made TB's for from a BMW K series, it had more than 170 hp from 1300 cc, from memory they were only 34 mm
Biggest costs can be things you don't expect. TB/Computer are easy to budget for. Pumps, swirl pot, regulators, fittings, return lines and such are the things that sneek up on you. If your car came as injection in the first place that can save a heap as you will have access to OEM parts. (Don't know what model Jag you have) A lot of stuff in good condition can be picked up s/hand from the street guys who upgrade based on it is the "done thing"
As Graham said selecting a second hand injector with the same power per cly as you have is a great place to start, just get them cleaned and away you go, by increasing cycle time (max 80%) and also fuel pressure you will be able to easily get the 50% more than standard, The Suzuki above had standard injectors still, standard power was 100 bhp.
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