Which type of label are you doing? Woven, satin, or tagless printed?
Average for woven is folded 1 1/2 inches long and 1 to 1 1/4 inches wide.
Average for satin (no fold needed) is 1 1/2 long and 1/2 to 1 1/4 inches wide.
Tagless could be as small as above but are generally larger like 2 inches long and 3 inches wide.
Size will depend on how much info you are putting on there and whether you will use symbols where allowed or text.
Here are the rules on labeling
Threading Your Way Through the Labeling Requirements Under the Textile and Wool Acts