Can a soldier be made to do a height and weight, while on convalescence leave after having leg surgery?

It's the difference between the letter of the law and spirit of the law.

Yes anyone can be made to do height/weight when needed.

But if someone is on convalescence leave due to a surgery and they can't move around there should be a side bar conversation with some leadership to get this sorted out. I recommend the individual reach out to the Unit medical folks for support.

Can they? Yes. Will they? That is up to the Command. I've never seen a Command pull a Soldier back from Leave to conduct HT/WT....but stranger things have happened.

You mean Return to Duty (RTO) by the hospital that placed the soldier on Convalescent Leave?
Questions and Thoughts:
1. What organization placed the soldier on Convalescent Leave? That should be the Command directing further actions. i.e. where to report to next.
2. Is the soldier returning to the hospital? If so the soldier is still under that doctor / organizations place of duty.
3. Is the soldier returning to his unit? That normally is determined by the extent of the soldier's injuries. If so the returnee's Command only 'assumes' (you know how to spell that word) control upon the soldier's RTO.
4. Is the soldier RTO going to be another unit? If so the returnee's new Command upon the soldier's RTO.
Reference:
Chapter 12, AR 600-8-10 See "Command" 1 thru 4 above: Convalescent leave is a non-chargeable absence from duty granted to expedite a Soldier's return to full duty after illness, injury, or childbirth. Organization Commanders may approve requests for up to 30 days, but requests which exceed this, or are in addition to this, require approval at a higher level.
Here's a quick study Link:
https://www.armystudyguide.com/content/army_board_study_guide_topics/leaves_and_passes/using-convalescent-leave.shtml