"Laspistol" isn't, of course, one weapon. It describes simply a compact, one-handed weapon based on the laser technology of the Imperium (if you check out the Tau of PO, then you'll see my personal take on Imperial laser weaponry).
So, yes, there are weapons as large as a Mauser or Uzi, but also as small as a Walther PPK. Digital laspistols are as small as a finger ring, yet just as powerful.
Don't take the tabletop weapons in the 28mm range as a true guide. They are deliberately larger than life to meet the needs of the tabletop gamer (the ability to identify quickly and at a distance what weapons any given model is holding).
By way of example, this old 54mm model is wielding a 28mm revolver and it looks perfectly in-scale:
R.