Subcommand of [array], returns the number of keys in the array given by name.  Example:
 % array set arr {foo 1 bar 2 grill 3}
 % array size arr
 3

It has been pointed out in the [Tcl chatroom] that [array size] is almost double as slow as ''array statistics'' which returns much more data (try it to see..) If the speed matters, replace calls to [array size] with
 lindex [array statistics arrname] 0

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