Here's a gantry that was designed for dead lifts of my old diesel engine/generators that weighed up to 2 tons. I know the gantry can do more than that. It's 7.5' tall, 4 feet wide and has base legs 4'. I have used it for rolling things by putting wheels under it but, use very cautiously with the weight very low. The biggest problems you face are swaying of the weight and twist/stress on the struts. If something fails, it crumbles.
In physics, there's something called a free-body diagram where you draw the weights and supporting members and calculate the forces to determine the basic geometry. Then you need to calculate the forces on the welds and bolts. It's about ideal to have base legs that are about 2/3 (or more) of the maximum height. But when it comes right to it, the leg length really depends on how it's used.
Take a look at the gantry that Harbor Freight sells. The geometry is pretty good but due to component quality I don't think it's good for everyday, industrial use. Also keep in mind, it's almost certainly made of alloy steel and thus the seemingly thinner gauge metals.
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