Rear derailleur cage length is determined by the "total teeth" ( (big chainring - smallest chainring) + (biggest cog - smallest cog) ) you need to handle. For a 44/32/22 cranks set and a 11x32 cassette that number would be (44-22) + (32-11) = 43 teeth.
The manufacturer specifies the total teeth every derailleur model will handle so you just do the calculation for your planned gearing and see which model meets your requirements. That said, the longest available cage will work fine even if it's capacity isn't needed. The only downsides to a too-long cage are a very slight weight penalty and less ground clearance. There are those who claim a shorter cage shifts faster but I've never noticed the difference.