Preferred techniques for smaller/lighter people?

I'm a 50 year old 5'6" 155 lbs. blue belt who is constantly training with guys much younger and significantly bigger than me. If you just started a few months ago, I wouldn't worry about finding your own game yet. You should be working the fundamentals with a heavy emphasis on defense. Heck, I'm 4+ years into this and I'm still doing that. In the beginning, focus on survival and escape from the major dominant positions: back, mount, cross side. Don't worry about all the variations of those positions yet, just try to develop a good A/B game for escaping. You A game should probably be getting really competent at elbow escapes from all of those positions. As you get good at those major positions, you can start to develop defenses and escapes from variants of them, like KOB, N/S, modified scarf, etc.

Next for you probably should be getting good at guard retention both in the open guard, but also in half guard since a lot of your escapes will bring you to half guard. I think closed guard isn't a good idea to use for smaller grapplers against larger grapplers, but in the beginning, you probably should avoid it anyways because you need to develop your guard retention skills. If you get competent at escapes and guard retention, you can sorta choose what fundamental area to focus on next, but it should probably still be fundamental/universal BJJ. Expanding your game to a positionally offensive half guard game, or developing sweeps from open guard positions might be good choices. Standing mobility passing might be another good choice. Basically, what I recommend, as someone similarly situated, is emphasize defensive fundamentals that everyone should know regardless of size and not worry about finding your own game yet. Maybe someone with more experience than me can chime in with some better advice, but that's mine.

Open guard is usually something that smaller grapplers can use effectively against larger opponents. I don't know what your flexibility is like though. If you don't have much flexibility in your legs or back it might jot work as well. No matter your flexibility, improving open guard retention will help you. You might also want to check out matches where smaller people beat larger opponents. This happens fairly often in Bjj, so you can get an idea of what moves work against people with a size advantage.

Everyone game development is unique, but I guess butterfly guard and shin-to-shin are good open guard options and half-guard (without shield, really under oponent) or deep half guard.... when I face bigger opponents those are that helps me a lot to handle pressure... (always remember, fall 7 and stand up 8 mindset - takes time to adjust positions, so at first attempts mistakes are common and your main job is realize the micro adjustments needed to improve those skills).

Looking back if I honed in on establishing either inside sankaku or arm trapped back mount. And than finishing with inside heel hook or rear naked choke respectively I believe I would have done much better against bigger and stronger opponents. As those are the positions and techniques that I have observed provide the greatest leverage.

First of all thanks a lot for your help and support, lots of interesting thoughts and ideas. Keep em coming ;-)

I'll have to look up some of the techniques first but am definitely going to try to test those when rolling.

Really looking forward to the journey and developing my own game slowly over time.