Hi George,
The previous answer is correct.
A problem that could occur with too large a slave cylinder is insufficient range of movement for accurate adjustment. This could lead to a more quickly worn out throwout bearing, if adjustment is such that the T.O. bearing drags. At the other end of the adjustment range, the clutch might not disengage all the way and shifting would be more difficult, could lead to gearbox problems eventually.
I may be wrong, but I thought .70" and 3/4" were the two sizes of clutch slave cylinders used on all the TRs, with the switch to 3/4" coming in the middle of TR6 production. I seem to recall TR2 and early TR3 used a .70" with a differently threaded hose fitting, but otherwise the same as used on later TR3/3A, TR4/4A, 250/5 and early TR6.
TR6 was switched to using a non-adjustable slave cylinder pushrod. That and a heavier-operating diaphragm type clutch (in use since TR4A) lead to some problems with short clutch/T.O. bearing life, probably are the reasons the switch was made to a slightly larger .75" slave cylinder.
Also effecting the geometry of the clutch release mechanism is which of the three holes in the release shaft lever is used to connect the clutch cylinder push rod. All TRs use the middle hole. Top or bottom will give incorrect range of motion and either make the clutch too heavy or give too little release motion.
I don't think there is much if any effect on the U-joints, no matter which clutch slave cylinder is used. Smooth shifts are the best way to help U-joints stay healthy.
I had a non-greasable set of U-joints in my TR4 for 25 years - just recently replaced with greasable type - but not really showing any wear I could detect, despite some pretty hard driving and a modest racing career. The only problem was re-occuring loosening of the 8 fasteners, which I hope is solved now with the upgraded bolt and all-metal-locking nut set I got from The Roadster Factory and installed with Locktite for added security.
Hope this helps.
Alan