The RTC have an excellent weight : length chart on their website which helps in estimating the weight of fish so that they can be returned nice and quickly and with the minimum of handling.
See:- Length/Weight chart
They also have excellent guidance on the handling of Spring fish:-
FISH HANDLING Fish should be handled with extreme care, especially those that have been injured or become exhausted whilst being played.
* Use a knotless net to bring the fish into the side of the River, if necessary
* Do not take the fish out of the water
* Do not hold a fish up by its tail or close to your body under any circumstances
* Handle the fish as little as possible and as gently as possible: fresh scales come away from the skin very easily and allow infection and disease into the fish
* Fish should be supported gently and upright in the water until such time as they swim away on their own
* Anglers must allow the fish time to recover before letting it loose in the River; this may take a long time, and up to 30 minutes