Going with larger tubing may not be the answer. Larger tubing is going be heavier and you will have to accelerate the added mass.
You might want to investigate making a stiff structure for your gantry - maybe even using smaller tubing, but using more than just one beam. Something like three arranged in a triangular manner, tied together with spars could be made lightweight and stiff. I don't know if this makes sense... There are lots of ways to get a stiff structure without adding a lot of mass.
Tubing is available in pretty much every inch dimension - 2X2, 2X3, 3X3, 3X4, 4X4, 4X6, 2X6 etc. Common wall thicknesses are 1/8", 1/4", 3/16" and many sizes of tubing are available with gauge thickness walls - check google for 'steel tubing' and you will find online suppliers and their stock lists, and quite possibly pricing, but you will likely get a better price from a local supplier.
Scott
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