It depends upon the species, but if the mudskippers are one of the smaller sized species, then the tank you mention should be fine as far as dimensions. The "land" portion is much more significant to the mudskippers than the water depth. Provided they can fully submerge themselves in water--which means it can be very shallow, say 2 inches (5 cm) depth at the deepest area, they will be fine. Providing a beach setting is ideal, where the sand (it needs to be sand) is sloped from the water up to being "dry." Logs on the "land" portion are good, as this is natural.
Mudskippers will remain out of water more of the time than submerged. They must keep themselves moist (much like an amphibian) and they will return to the water for a mouthful, as they "breathe" primarily by extracting the oxygen from the mouthful of water, replenishing it when they need to. So the sand can have lots of areas where is it sitting in water rather than being actually out of the water. Sort of like shallow puddles.
The feed on the land area, not underwater. I had one many years ago and every night I fed him foods like small chunks of frozen squid, shrimp, and similar (thawed of course), from my fingertip. Once or twice a week I put a very small clump of live small worms (tubifex back then) on one of the logs where I always fed him from--and he got used to that...at the first sound of the tank cover opening, he would jump into the water and swim to the log, climb out, and literally jump for joy. He got so excited a couple times that he jumped right out and I had to chase him over the floor.
Brackish water is mandatory. And keep the mudskippers on their own, no other fish,etc. Also, if you want more than one, get them together from the same source (tank); this can avoid introducing disease from different sources, something that happened to me and I lost them.
