I think the size will be 12288×4320 if you arrange it as 3×2.
Your monitors are ok for the HD 7850 Eyefinity 6.
I have never tried excel at those resolutions so it is hard for me to say heheh..
In regards to SLS means Single Large Surface which you create a single desktop out of multiple displays. Same as  you describe the 1 big screen/desktop and yes, your mouse should behave in the same manner as you are using a single desktop.
There are no stupid questions when it comes to a heavy investment as you mention.
We are here to help.