If it were mine I'd start with DCOE40 and 34mm chokes.

I agree with Steve's numbers. If you want to apply some science to it, the rule of thumb for Weber sizing is that the choke diameter in mm = (individual cylinder displacement in cc's X RPM at maximum power / 2500 ) ^ 1/2. And if you can't find your slide rule, you can see it in graphical form at

http://www.dvandrews.co.uk/ by clicking on "Selection and tuning of DCOE carbs". Andrews' chart is for 4-cyl engines, but taking the 3,453 cc displacement of a "3.5" BMW engine and multiplying it by 2/3 gives you the appropriate point on the X axis.

For a BMW 3.5, plugging 575.5 cc / cylinder and 5,200 rpm into that formula gives a choke size of 34.6 mm. But of course, Weber chokes are only available in integral mm sizes, so 34 mm is your closest, readily available choice (do they make 35's?). Andrews says that the ideal carb size is 1.25 X the diameter of the choke, so 1.25 X 34 mm = 42.5 mm: halfway between a DCOE 40 and a DCOE 45. Since 40's are typically lower-cost than 45's, and since you probably aren't going to regularly go as high as 5,200 rpm in a street car, a DCOE 40 with 34 mm chokes is probably a good starting point.