My preference is one that can report fractional inches. Unfortunately, most of these will report to 1/128ths inch increments, which is way too precise for marbles, IMO.
Vintage American marbles were sized in 1/16 inch increments so, for example, any marble that would fall through a 5/8" hole but would not pass through a 9/16" hole would be sold as a No. 0. Any marble that would pass through an 11/16" hole but not a 5/8" hole was a No. 1, etc. And I can think of no case where a 1/32" difference in a marble's size would definitively change its ID.