I have a 240 mm Rodenstock Sironar in a #3 Copal and it is a big heavy piece of glass. Make no mistake, it is a very good heavy piece of glass. I just would not want to walk very far with it.
I've seem the Schneider Symmar S 300 and it was real big. From what Ive read it's sharper than the compact Nikon 300M, but I realized I didn't want to get too large/heavy to start out with. Again, those comprimises ...lol
I have the heaviest one of all: A 300mm/f:4.5 Xenar in a #5 shutter. Weighs a bit over one kilo, won't fit most lensboards... I use it on a Technika III 5x7 camera, which has big lensboards.
I also have a 360mm/f:5.5 Tele-Xenar in a Compound #3 which I use on my 4x5" Linhof Color. This uses standard Technika-type lensboards, so I can't see why you couldn't use one on most cameras? The lens is rather large and heavy, but it's a lot lighter than to carry the Tech5x7 and reduction back in case I need a longer lens...