if the tractor is 11+ feet by itself, I think you can safely figure another 5' of fel, depending on if you curl the bucket for loading. If that 7' mower is a rough cut, like you state it is.. then it will be 7' plus tail wheel, figure 9' ... 7' rough mowers are usually single spindle, and thus lon, and heavy...
I wouldn't attempt to haul that machine as configured without a 25' trailer. A GN with a 20' deck plus 5' beavertail or some combination of the same like a 25+2 dove.. 28' straight with fold over ramps.. etc..e tc. This will likely be a 14k tankem.. though you could find it in a 21k tri axle, or if you have the extra cash, go for a 20k dual tandem, even if you tag it for less weight. ( depends on your area for what is leagal for tagging, weight and GVCWR )...
PS.. that's a good amount of iron, thus you need a good towing vehicle. I don't think I'd attempt it with a 3/4 ton.. 1 ton would feel much better...IMHO
here's a pic of kinda what I'm talking about.
soundguy