What tack welding size and spacing is needed in relationship to material thickness?

If tack welding is just for the purpose of holding the part in place then the welding tack size should be enough to hold it in place and should not exceed the final welding size. Spacing and number of tacks can vary depending on your requirement, but if enough space is available then try to avoid the consecutive tacks in the heat affected zone area of the previous welded tack. Usually spacing of 25 to 30mm is enough, increase the spacing as there is an increase in the leg size of the tack weld.