It is made for about 208 pixels but I find going to 225 hides the zone better. On the 13 foot screen it is about 16-18 inches wide. So yes it is small, my zoom I think is set at 177 after using the 1:1 pixel mapping in the source menu. I'll have to double check when I get home. The zoom is what determines how large the blend zone will be, but when using 1:1 you have to make adjustments and some menu's get hidden so adjustments will not ruin the mapping.
I find the image is more precise allowing the 1:1 and then just going with what it throws out at you
There may be other ways to do it but my physical location is set in stone for now I am not moving those damn things for a long long time !