Hi Matthew,
I reached out to one of our Senior Technical Artists and this was his response:
Landscape has can have so much to do
with the intended project that it is
hard to suggest a one size fits all
size.But for most of my landscapes, I went
with a 4033x4033 resolution and that
seems to be enough to have a detailed
playable areas and some ‘vista’ space.
Using World Composition can also allow you to get better resolutions and tile your landscapes as well. You can now have them loaded in via BPs and unloaded to keep optimization up.
Feel free to ask anytime you have concerns or issues and I’ll gladly help out! 
Tim
p.s.
Thanks for your kind words in your Forums post in the feedback section!