The bolt size is 1 and 5/16", same as a hub nut.
You don't need to remove this though to change the parts you want to change.

Yes you can just remove the top arm. You undo the 2 forward retainer bolts, as well as the 11/16th AF nuts. Then the pin slides forward into the engine bay and releases the top arm down.

It is more work than using the spring compressor, you really need to remove the radiator, and work is really tight on the passenger side as well. But it does solve a problem if you haven't a spring compressor.

_________________
SooperDooperMiniCooperExpertEngineering

All garage work involves equal measures of enthusiasm, ingenuity and a fair degree of irresponsibility.

If you drop into somewhere like just tools, they will sell you the socket for about 35 dollars. Well worth it, as said it can be used elsewhere on the car as well.

I would rather do it with the spring compressor any day, even if you need to move a few bits and pieces out of the way. The lower bolts are worse than anything sometimes to get out. Especially in a rover with all the accessories in the way.

_________________
SooperDooperMiniCooperExpertEngineering

All garage work involves equal measures of enthusiasm, ingenuity and a fair degree of irresponsibility.

On a 97 model mini - the job to replace the cones is a right PITA.

Access to the drivers side arm shaft is impeded by the AC condenser and brake booster - let alone the clutch housing. As mentioned, passengers side, the radiator is in the way.