The problem I see is that I don't necessarily see the Bolter as an assault rifle, more like a quick-firing grenade launcher. a .75 round is really rather large, just under 20mm for metrics (like me), and the bolter round is an explosive thing. So I see Bolters less as assault rifles, more as scaled-down grenade launchers.
A big thing is the size of the round. Taking the .75 calibre (because I see that as more realistic), we have a round that (taking the garden variety 40mm grenades of today as a base) is 20mm diameter, and about 50mm high. That's a big, squat round, however you look at this, and the size of the magazine supports this somewhat. It's just not big enough to take a conventional rifle round (Very long vs small diameter). The barrel is also very short, suggesting a weapon that is not designed for accuracy, but just to 'point it in the right direction'. I remember reading somewhere that the Godwyn pattern bolter is smoothbore, but I can't be certain, but if it is, it reinforces the issue of the bolter really being a small grenade launcher.
Now, looking at the size, this all makes things a lot easier to understand. We can make a good estimate at the magazine size. The sickle magazine is meant to take 20-30 rounds. Now, stacked alternately like in modern firearms, and using the smaller number, this gives us a rough height of about 35cm, depending on how acute the stack is, and a width of about 5cm. That's big, and I'd say, roughly consistent with the size of the magazine on the models. a 30-round magazine has us sitting at about 50cm long magazine, which is just far too big, in my head.
Now, onto the internal mechanisms of the Bolter. If it uses a standard percussion cap firing system like modern firearms, we need somewhere for a firing pin and spring assembly. Then we need a holder for the shell, springs to push it into the breach and then pull back the empty case after firing. That's large, and I'm guessing takes up most of the rear mass of the bolter, considering the size of round and firing stresses. If it's electrically fired, we need a lot less working parts, and the size of the working parts will be reduced, so that they take up less space inside the weapon.
Then we need all the gubbins included, like genetic grips and targeting systems, taking up yet more space inside the thing, almost too much space in my head, but GW don't do 'tru-scale' that coincides with the fluff, so that's understandable.
All in all, for the bolter, as a rough guess I'd say you're looking at a barrel length of about 45-55cm (working this out in my head), a body width of about 7-10cm and a height dependent on the magazine, but empty, I'd hazard a guess at 35cm high. Sounds very squat and fat for an assault rifle, but sounds about right for a 20mm high-velocity grenade launcher.
BUT, on the other hand, you could take all of what I've said and throw it out, because I don't live 38,000 years in the future and I don't know what kind of tech they have!
Hope this kind of helps...