I’ve got a 2 yr. old Garmin 9” Echomap UHD, and am thinking about adding a Garmin Livescope. Bass Pro has them on sale now. My question is: is a 9” screen wide enough to use the Livescope effectively, or should I go larger , like a 12” screen( for another $1,500)?

Can’t answer your question per sey, but if you’re willing to go to a 12” then do it. If you were to go with the 9” and aren’t happy you’re SOL. I’ve never heard anyone say I wish I had bought a smaller screen. 

Yes!!! It works just fine.  I use 2 9" units on the bow. Fished a 7" for almost 3yrs prior.  The 9" is a perfect size. Only downfall you loose the ability to network units unless you get a data hub since they only have 1 data port opposed to the 10 and 12s have 2. You'll be fine. 

I don't have Garmin units, but I do have a 9" Lowrance up front on my boat and it's a good size for a dedicated FF sonar screen. I do have plans to add a second unit though because split screening shrinks it down too much for my taste. If you can swing it get the 12 and then you'll have the extra port to network everything together.