In practice it is the leech length and the corresponding pole height that matters and small kites with long poles look a bit odd but set perfectly satisfactorily and are helpful for small/light crews when it is windy. Been there, done that.
Have had quite a lot of experience sailing a Merlin with crews who were young, small, light, inexperienced & a combinations of that mix. I have also sailed Merlin’s by myself...
My experience was not to bother with the smaller spinnaker as the complications of having two setups outweigh any advantage. My solution was to make the handling of the larger spinnaker easier with a careful set-up & things arranged so the helmsman is able to take on some of the work load usually associated with the crew especially where strength is required.
To this end my boat had two sets of spinnaker sheets both slightly longer than normal with two very carefully positioned stopper knots. The boat was equipped with ratchet blocks, two sets of cleats, twinning lines employing blocks (all Harken fittings) an additional lower eye on the mast & the whole system very carefully engineered to ensure free movement of all parts.
The length of the sheets is actually the key to all this together with the positioning of those knots.
In my experience nothing frustrates young &/or inexperienced crews quicker than wrestling ineffectively with the spinnaker pole. As with most things there are some drawbacks but in my experience this system works enabling the larger spinnaker to be carried on the broader legs whilst perhaps two sailing the close reaching legs earlier than one might otherwise do but often this works anyway & in such conditions preserves the crew’s energy levels.
Spinnaker sheet No1:
This set of sheets makes for the easiest, safety first management of the spinnaker. These sheets are long enough so that the spinnaker can remain completely within the shute with the pole set but with the snodger off, twinners off, & the knot in the windward side of spinner sheet hard against the ratchet block. The distance between knots is just a little more than the distance between the ratchet blocks. This arrangement allows the pole to be set independently of the spinnaker. The drawback to this is, of course, the longer sheets which takes longer to sheet in, are more prone to tangling & more prone to going under the bow. The delay in getting power on & the increased risk of a tangle are little compared with the delays whilst the crew struggles with the pole attached to a flogging spinnaker. The risk of the sheet going under the bow can be managed by the helmsman pulling in the slack as the crew releases the pole from the mast. With this system it is possible for the helmsman to drop the spinnaker single handed thus reducing the risks associated with being caught out with a young crew in a squall. This arrangement also makes it possible to sail a Merlin & utilize the spinnaker single handed.
Spinnaker sheet No2:
This set of sheets makes a good compromise between performance & ease of use. These sheets are shorter in that the knots are positioned such that the knot in the windward sheet is hard against the ratchet block when the pole is set with the snodger on, twinners on & the pole about 150-200mm off the fore stay in a strong breeze. The distance between the knots is the same as before. There is still some disadvantage with the length of the sheets delaying power on & the time taken to do the twinning lines but once again reduces the risks. In lighter winds one can release the twinning line if it is felt the pole is too far off the forestay.
Additional eye on the mast gives the crew an easier option if they miss the higher one. Whilst it is preferable to use the upper eye having it on quickly is often more important & certainly better for crew moral.
Second set of cleats are found on many boats anyway & allows the windward sheet to be easily sheeted & cleated by the helmsman. The idea of the two knots is that the leeward knot stops the leeward sheet uncleating the windward part of the sheet whilst the sail is flogging prior to the crew sheeting it in. The second knot also prevents the situation where the spinnaker fills with the continuous sheet taught & the leeward part becomes shorter than the windward part… very difficult for an inexperienced crew to sort on a windy day in the time available.
Twinning line (continious) just long enough so that on a run with both off they have no or minimal effect on the lead of the sheets & the blocks attached with a reduced splice to ensure the run freely out of the eye on the gunwale. That is also just long enough to flick the leeward one out without uncleating the windward one.
This system was originally devised & evolved so I could sail with my daughter (10 at the time) but subsequently enabled me to sail the Merlin with a variety of crews. I very much enjoyed that season sailing with my daughter & hope it helps you too.