That's cutting it pretty close. I'm in that same boat. I have two about that size and their habitat is right on the cusp of being too small. Hopefully it will last until next spring when I'll build them a bigger one or let them out into the big boy yard.
Its a bit small for that size leo. Outside is the best for a leo that size so they can graze and get lots of sunshine, Weather permitting of course. I would up to a 8' by 4' if you could for an indoor table. Any reason you dont have him oustide?
There is no real rule for this. A bigger enclosure is better. It's not that a small one will kill them, but a bigger one will allow them more room to exercise and explore. It will give them a larger area to thermoregulate. Help to keep them from getting fat, etc...
I also say it is too small. For night time it would be fine for quite some time. However, he should have way more room then that for day time grazing and exploring. Can you at least put a second floor on it with a safe ramp going up to it? Possibly join two 50 gallon tote boxes together, with a hole and tunnel to join the two together. Just some ideas to expand his small enclosure.
Not sure there is a how to or step by step, but check out the enclosure threads and you might get some good ideas and I believe there might be a few that shows or gives a few instructions.