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You can probably look at the Dreadnought Plasma Cannon and Devastator Plasma Cannon for reference. In general, the dreadnought version tend to be about 10-15% bigger than the marine version.

However if you're making rifle versions, maybe use the marine size since it would look more appropriate to be hand held rather than be a fixed weapon.

Scaling in 40k is, to be frank, pretty stupid.

There's the aforementioned Dev/Dread plasma cannon comparison but other examples abound.

The size of the Razorback's heavy bolter compared to the ones lugged around by Devastators is similarly silly.

The lascannon that IG weapons teams use is ludicrously big, and this is especially confusing when you compare how much smaller the ones used by Space Marines are. Shouldn't the power-armoured super-humans be lugging the heavier version around? And even so, the IG one is still just...stupidly big.

These scaling issues carry over into art, too. I forget which piece it is exactly, but I remember one painting in the new rulebook stood out to me because the background features what looks like an Icarus autocannon mounted atop a tower...but judging by the humanoid figures in the distance nearby it, a Space Marine could easily jump down one of the barrels.
Just...what?

And don't even get me started about vehicle scaling...or "true-scale"...it's not just GW that has this problem.

In short, as long as you can devise something that "looks right", just go with it.

Well miniaturising stuff can be hard sometimes and if you need to churn out Guard weapons by the million it makes more sense to do it with the easy ones.

On topic, the rule of cool wins this one I think. If you want to make a dread sized rifle then muck about until you find a size you like the look of and go with it. There are so many marks of the different weapons you can get away with anything.

To add to the confusion of scale, there's also the Leman Russ and Armored Sentinel sizes of plasma cannon - another two sizes that have no parallel between them and the Marine versions.

Look at any tyranid weapon. Like... any biocannon. Compare the size of the hive tyrant, to the warrior, to the ravener, to the tyranocyte/sporcyst, to the harpy. All different sizes for the exact same profile.

GW scales it's weapons to the unit they're fielded in. A lascannon on a big tank is bigger than one carried by an infantry model. You should follow suit, and make your weapons match your units. There's no such thing as a standard pattern imperial weapon so go to town with them!

I'm fairly sure I saw a dreadknight once that was carrying a rifle weapon in it's arms. It can be done, not sure how you'd do it though, bit beyond my skill!

The IG weapon team Lascannon is bigger than the one wielded by a Space Marine due to the former being implied to be the exact same one off of the tanks (with a portable battery rather than one linked to the vehicle's engine) while the one wielded by a space marine is a special "man portable" version, hence why the superhuman one is smaller than the one used by the common man. The overall weight, I'd imagine, wouldn't be that different, hence why two guardsmen can still carry it around. The difference is that it needs to be set up to be fired, while the marine just carries it on his shoulder.

I suspect it'll be awkward. Dreadnoughts, even Contemptors, just aren't shaped well for holding rifle type weapons.
Have you considered a Devastator style rotary heavy bolter? (essentially a mini Vulcan bolter)