Many here will not like the idea of beam fighters, and there are good reasons for that as they really get shot down easily in small groups by enemies mainly missile based ships and/or cannot take much from dedicated beam ships either. However, if you consider that beam attacking is basically a strategy that involves getting as fast as close enough to the enemy before he can fire missiles back, it is obvious that small stealthy, yet lightning fast fighters must just be the ideal beam attack craft - so if the strategy is valid at all, then with this.
I personally have successfully tested larger beam-fighter groups of 20, 40 and 80 against the enemies of swarm and precursor (no NPR yet though), and every incident was a huge success - sometimes even surprising. If you always remember to keep the thermal dampening tech at the niveau that the current engines are, then most fighters will only be spotted around 3-5m kilometers in distance.(but beware of good actives. needs calculation) That still leaves the enemy time to shoot some missiles, but firstly can a sizable fighter group defend against that pretty good, and secondly even losses aren't too bad, as the enemy only slowly retargets after firing all out on simply one fighter in overkill mode, - so there is usually enough time.
The reason why beam fighters have such bad reputation though might be because they always get deployed in underwhelming numbers. The philosophy here is different from missile bombers, where a few can already have great effect (but exhaust their power quickly). If you send only 3-5, or even 10 fighters against an enemy destroyer, then it is 1.5-5kt against 7 or 8kt .... - no wonder the outcome wont be without losses. Unless you have serious tech advantage, you must be sure to meet enemies with either slightly lower, but best more than equal force. (I used for example 40 beam attackers (20kt) to hunt down 14kt precursor cruisers or pairs of destroyers, and managed to destroy them without own losses after just one salvo of point blank fire .... at TL1!*)
*= Well, except engine power factor, which was full 3.An effective strategy I have found in beam-fighter design, is to completely forego the idea of range on them, and just always attack at point blank. Your beam fire control can be really small through this, which saves space, and you are sure to do maximum damage, as well as hitting 100% of the time thanks to the enormous pilot bonuses that fighters receive (they even allow to ignore good ecm'd ships). On top of that it also saves costs on a laser, because you can just keep using ultra cheap downgraded infra-red for fighter weaponry.
...My face when I noticed the difference that wavelength tech makes.^^
Otherwise, the best ranges on fighters are usually around 40-120k, so really not that spectacular, and crossable by any fighters in 5 seconds at the corresponding TL, so no win in time here for sure. Range on beam-fighters would only make sense if you seek to outrange either enemy beam ships or FACs.
Beam ships will never work with that, except the stationary beam bases. Anything else usually has a larger caliber than a fighter could mount, and that settles it already.
With FACs I could be wrong, but the only ones with beams I've seen came from swarm so far, and they seem to have either 60 or 120k range - both doable. As specialized anti swarm fighter, range is actually very plausible, but as an all-mission vehicle, I would recommend only point blank.
Now for design - I doubt a beam fighter can be done smaller than 400t, except if you use gauss, which I wouldn't recommend because of the gruesome useless hit-rates of smaller cannons. Even a miniaturized laser is still 100ts though, and with the beam control and generator surely 150. An efficient fighter should have around 40% engines at least, which counts double for beam fighters, because, again, beam attack is all about getting in as fast as possible. So with that it is already 300ts, and with the adding of smallest possible engineering, life quarters, armor and some fuel, there surely is another 50t=100t with engines to come.
I would like to see any design that can do it lower though. I personally always look to go to 500 directly, because I like to have some modular extras on them like more armor, a stealth suite to also hide from actives (which is needed more against higher TL enemies), or my favorite...
..Even if you might look out to miniaturize for bigger fighter loads, but if your tech level outranks the casual enemies a bit, consider adding a small ecm on beam fighters. The thing really enhances survival chances greatly on ships that are already hard to hit due to speed, so additional 20% or more might make your fighters up to immune at some point even. (my best test was against swarm facs here, where I could just fly 40 fighters directly on top of a stack of 80+, and they couldn't do anything about it anymore. from there I just shot them down with the 'different targets' option automatically. the most efficient anti swarm weapon I found yet, but you need to have a great technical edge of +3-4 levels, so 20-30% offset through small ecm)