The limit on the biggest cassette you can use is obviously defined by your rear derailleur. Most Shimano 10-speed road rds have a 30T big cog limit but will accept a 32T in most applications. Any Shimano 9-speed MTB rd will index properly with 10-speed shifters and they will take a 34T or slightly larger cog. They will also "wrap up" a lot of chain so you can use wide ratio cassettes and triple crank and are really better than a road rd plus a goatlink.

The best thing about the Gevenalle shifters is the front is friction and very agnostic about the crank and front derailleur you choose.

A 26x34 gear is a 20.6 gear-inch low and 24x34 is 19 gear-inches and either is probably enough for most touring riders.