Tamaño is the physical dimension, or extension, or magnitude of an object. It's used everywhere in that meaning. Indirectly you may use it for clothes, but it sounds pretty rough that way. Some outfits, like a jogging set, may be asked by tamaño.
Talla (stature, even figuratively) and talle (waist, waist size, figure, appearance, outline, ... by extension any measurement taken with tape, like neck size) are used for clothes. Talle is the common word here in Argentina tough número is used for footwear.
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