Not sure on the size, but I can tell you it's a bad idea. I'm running a solid front mount and it's ended up tearing up my rear mount something bad. I imagine a solid rear would do the same thing to the front. Try injecting some polyurethane in to the air gaps to just make it stiffer.
Yep, solid mounts are bad IMHO on anything other than all-out race cars that use the engine as a structural member of the car. Engines are supposed to move to absorb shock, especially the enormous loads imposed by the drivewheels (both reaction torque from throttle inputs and from braking as well.) Without such movement, all of the shock is directly transmitted to the comparatively fragile transmission housing and engine block points unabated. Transmission housing cracking isn't unheard of due to solid mounts.
If you really need to stiffen up the engine mounts, try to maintain at least some compliance. Look into polyurethane inserts or modify your own mounts by filling them with the stuff.
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