Does anyone have an advance for deciding on the dropper length
As long as you can fit and afford basically, there’s a few folks that don’t mind a short dropper but by and large longer is better. It’s pretty much the equivalent of having a high gear range- it’s fine for a while then you want to use something you don’t have and it all comes apart.
Unless it’s for an XC bike I’d say don’t mess around with 100mm. I had a 100mm gravity dropper for a long time and it was, well, semi-useful. Good for simple riding. But the difference between that and a 150 is huge if you’re into riding harder stuff. Course, longer tends to add cost.