Open

"open your arms" is OK; it may be used literally or figuratively (in the latter case meaning to welcome or willingly accept something).

"open your legs" is also OK in terms of English, but should be used with care as it may have sexual connotations.

Can I say to someone standing up, "Open your legs at the shoulder width"?

lucas21cCan I say to someone standing up, "Open your legs at the shoulder width"?

You have not told us the circumstances; as GPY has already explained, it can be quite rude. In any case, the phrase would be 'spread your legs to shoulder width'.

Or "Stand with your legs shoulder-width apart."

The situation is like this. I am an instructor and there are many people being about to do exercise. Can I say to them, "Open your legs at the shoulder width and folloy my motions" so that I can make them do some stretches first? Or, under this circumstance, does the usage of 'open' sound awkward?

It seems that you do not read the answers we give you, lucas.

I am sorry that you feel like that. After giving you the exact circumstance that I did not told you, I just would like to confirm whether 'open' can be accpetable or whether it still sounds rude under that circumstance.

lucas21cI just would like to know whether 'open' can be accpeotable or whether it still sounds rude under that circumstance.

It still carries rude overtones.