Yes a 6" Kurt is too large in my opinion.
I have a RF style mill and went with a 4" Glacern vise. Things I considered were the size of my table (24" x9") A large vise is going to hang off the front of the table a long way or it's going to take up Y-axis travel because it is overhanging the rear of the table.
Also a 6" vise is tall and takes up valuable Z-axis travel. I believe you have a maximum of 13" of Z travel. Subtract about 4.5" for the height of the vise and you have a little over 8.5" of Z height remaining for tooling. Add a collet chuck and an endmill and there's not going to be a lot of room left.
Buying a 6" vise in case you upgrade is not a good plan because it will limit your capability with the mill you have now.
As far a brands go, Kurt is obviously cream of the crop, the 4" is more than the 6" but look at a 4" Glacern. They are excellent quality.