Would CdG chance with RN building carriers at the same time? Especially would they swap to using the larger civilian dock over the bay if RN ships are going to be that much larger than CdG....!
When you talk about civilian dock, I'm assuming you're talking about the Saint-Nazaire "Chantiers de l'Atlantique", which have the biggest dock in France. OTL, the CdG was constructed in Brest's Arsenal (DCN Brest, now Naval Group). You can note that the CdG is roughly 4 meters shorter than the Clemenceau, which was also build in Brest. The Brest's Arsenal has also institutional knowledge in handling nuclear power (it was already refitting the French SSBN). SO, it's possible that Saint-Nazaire get to construct the second carrier (if there is one), but I don't think the first can escape from Brest's Arsenal hands. (And that's without counting that the French President is a Socialist.)
Can the new French carrier(s) be (slightly) bigger ? Yes, specially if the French want E-2C from the get go (as they saw AEW was maybe THE biggest game changer in ITTL Falklands naval campaign). So, it's possible to increase the size of the carrier(s) from 261.5 to 265 meters and from 42,500 to 47,500 (metric) tonnes, if the MN can convince the politicians that they need 40 fighters, 2/3 E-2C and 2/3 helicopters for the air group.
The biggest problem with an increased size and tonnage in France is the Toulon dry dock (used for refit), but the one I'm proposing is manageable with a (slightly) bigger reconstruction of the dock than OTL. If you go over a certain size, maybe 60 000 tonnes, you need to reconstruct half the docks in Toulon, which will cost as much as one carrier.
As for comparing French and British carriers capacities, it's difficult. Part of the tonnage advantage the British carriers will have (55,000 against 42,500) will be eaten by ship's fuel, not by aviation fuel, spare parts and munitions (the 2 gives the extra capacities). Both are announced at around 40 aircraft, so the advantage of the increased tonnage of the British carriers is also limited.
In fact, I think the major capacity break through with the CATOBAR is when one carrier is big enough to launch and land aircraft at the same time. But, neither the French nor the British break that barrier.
So, all in all, the British carriers have slight advantage, specially in high intensity warfare (they probably still have bigger storage capacities for aviation fuel, spare parts and munitions).
Note : the 2nd Clemenceau, the Foch was built in St.-Nazaire.