Jabez Wilson describes both his assistant and his assistant's accomplice as "small." This is intended by the author, Sir Arthur Conan Doyle, to establish that it would be easier for them than for larger men to dig a long tunnel, because they wouldn't have to remove as much dirt. Also, their small size would make it easier for them to scramble around inside the tunnel, both while they were digging it and while they were hauling all those French gold coins back to Wilson's cellar. Here are the pertinent words from the story Wilson tells Holmes:
“There was nothing in the office but a couple of wooden chairs and a deal table, behind which sat a small man with a head that was even redder than mine. He said a few words to each candidate as he came up, and then he always managed to find some fault in them which would disqualify them. Getting a vacancy did not seem to be such a very easy matter, after all. However, when our turn came the little man was much more favourable to me than to any of the others, and he closed the door as we entered, so that he might have a private word with us."
This occurs when Wilson is being interviewed for the post with the Red-Headed League by Clay's accomplice, who calls himself Duncan Ross. A little later in their initial meeting, Holmes will ask Wilson:
“What is he like, this Vincent Spaulding?”
“Small, stout-built, very quick in his ways, no hair on his face, though he's not short of thirty. Has a white splash of acid upon his forehead.”
John Clay, alias Vincent Spaulding, is small but well-built for tunneling. He is husky and agile. The whole business of digging such a long tunnel is one that the author has to make the reader accept. It must be a least a full city-block long. There would be some danger of cave-ins, and they are not using wooden beams as buttresses. Therefore, the less dirt they would remove, the less danger there would be from cave-ins. All the dirt must have to be hauled back out through the tunnel and dumped in Wilson's cellar. It's a good thing he doesn't go down there, but Conan Doyle has forestalled that possibility by characterizing Wilson as old, fat, lazy, and apparently suffering from high blood pressure. He is also a heavy snuff-user, which undoubtedly affects his breathing. He would be unlikely to climb down steep, dark cellar steps out of mere curiosity. And if he did so, he would get murdered with a shovel and buried in his own cellar.