I’m not sure what country you are in but DXL in America probably has the sizes you are looking for.

You don’t need to use the mod manager, you can manually install it. You are also playing a Bethesda game, you are already playing Russian roulette with stability.

There is probably a mod that will re-enable achievements, if not yet, there will be soon.

Edit

It looks like this may do it, though I haven’t tested it as I don’t have the game. https://www.nexusmods.com/oblivionremastered/mods/125?tab=posts&BH=2