Hebs really need to be 16 months old before they go for slaughter. You won't get the same soft first fleece look from them, even if the fleece has grown back. So you have a quandary - send off as babies before November of the year of birth when the carcase will be tiny, or wait til they are big enough to eat and have rougher, less curly fleeces. In fact the first year fleeces will have to be dipped in water (just the wool, not the skin) then air dried on the line in a stiff breeze, to get that lamby look back. The tannery can brush them but they look rather daft!
To estimate the lamb's weight, get your bathroom scales, standing on level concrete, weigh yourself, then weigh you holding lamb, do a little sum and you have the lamb's weight. Halve that and you have a rough idea of the carcase size on the hook. 14-25 kgs is fine, but 9 kgs is too tiny to be worth it, and will not have grown much muscle by then.