It is a natural consequence of the majority of Sherlock Holmes stories being told in the first person by Watson, that descriptions of him are few and far between. The text must rely on people either talking about Watson in his presence or describing him to himself. Conan Doyle approaches this ingeniously and obliquely in The Adventure of Charles Augustus Milverton, where the following exchange between Lestrade and Holmes occurs as Lestrade describes one of a pair of criminals
'a middle-sized, strongly built man - square jaw, thick neck, moustache, a mask over his eyes.
'That's rather vague,' said Sherlock Holmes 'Why, it might be a description of Watson!'
'It's true,' said the inspector, with much amusement. 'It might be a description of Watson.'
While this is classed as being a 'vague description' by Holmes, it is in keeping with the view we get of Watson as an older man in 'His Last Bow' where the third person narrator described a disguised Watson:
'A passenger sprang out and advanced swiftly towards him, while the chauffeur, a heavily built, elderly man, with a grey moustache, settled down, like one who resigns himself to a long vigil.'
The passenger and chauffeur are Holmes and Watson, who is also later described as 'thick-set'.
All of this may seem a far cry from the thin as a lath, nut-brown Watson we are introduced to in Study in Scarlet
An acquaintance Stamford remarks on Watson's appearance:
Whatever have you been doing with yourself, Watson?' he asked in undisguised wonder, as we rattled through the crowded London streets. 'You are as thin as a lath and as brown as a nut.'
It must be remembered that this was a Watson who had been dispatched home from the 1880 battle of Maiwand in the Afghan War as a result of his injuries and takes place thirty four years before the events of His Last Bow.
So there is not one description of Watson which prevails throughout the stories, he begins as a young man, fit until injured in battle, we know he later played Rugby, this is referenced in The Adventure of the Sussex Vampire, but as I'm unable to find specific reference to the position he played there is a limit to what that indicated about his build, although it does indicate that he had been an active sportsman.
No story gives his height in feet and inches, but from the Lestrade description we can conclude that he was neither notably short, nor notably tall. As a young man he was thin and fit, engaging in active sports, then he got a little wider with the advance of years,, as so many of us do.
It’s possible that many people mentally tag him as ‘sidekick’ and diminish his stature accordingly.