Problem is, small front chainring is tiny on a triple, so the chain ends up really short to avoid it touching itself at the rear derailleur when on the smallest cog.
I mean, I can't see the situation when I'd ever have the chain in the small chainring and the smallest cog, but there must be a way of having the chain length so you could if you wanted but also have it on the big ring and largest cog (which I probably wouldn't use either).
Size it big/big plus enough to take up the derailleur as mentioned and you can accidentally go big/big without exploding your drivechain when you forget what gear you're in going up a hill, if there's a bit of slack when you accidentally go small/small you won't break anything.
Don't cross-chain.