Literally not an issue, just make it wider and you're good to go. The only affect is the dry mass goes up, which like I said isn't ideal, but as SpaceX has clearly shown there's really no issue with building a bigger rocket unlike what those Boing jews would tell you. Elon has already said that after Starship is operating the next step will probably be an 18 meter diameter (but still methalox) vehicle, Big Chungus. As >>10991602
put it a big low density fuel tank means you can actually get more surface area per unit mass, considering the payload mass does not increase in step with the tank volume if you switch to hydrolox.
Overall switching from methalox to hydrolox has the following effects; More expensive for a given total propellant mass, because the tanks need to be much bigger and bigger does cost more, though not prohibitively more. Larger vehicle per unit payload, so aerobraking gains an advantage. Difficult to store hydrolox in space for long periods, requires active cooling, that's added cost and complexity if you want that (all but impossible for a lightweight system to store hydrolox on the surface of Mars, deal-breaker for SpaceX). Specific impulse goes up, increase in performance (even though wet-dry mass ratio goes down, it's still an improvement, just not as much as a 1:1 Isp increase with the same density). Harder to make a high thrust engine, easier to make a high efficiency engine (thrust doesn't matter so much because total mass for equal performance is lower, but is still important).
To put it simply, in my opinion a reusable two stage launch vehicle could have been done a long time ago using hydrolox's Isp to make up for some of the drawbacks and shortfalls of technology of the time. This vehicle would be capable of placing large amounts of payload into low Earth orbit for cheap compared to any modern launch vehicle, but would not be capable of long term propellant storage.