The species commonly sold as the indian mudskipper is Periophthalmus novemradiatus, which does max out around 4" or so. I don't think I've ever seen one listed as B. boddarti for sale. The other commonly sold species is P. barbarus, commonly listed as the Atlantic or African mudskipper. These get a little larger (6" or so) and are less tolerant of conspecifics.
In any case, I would look for a 20long minimum for a trio of the indians, since as jpappy mentioned, you'll need both land and water areas which can have a pretty limiting effect on how much water volume you actually end up with.