I use the mod all the time, it’s very simple but it works pretty well and is very customizable. If you use one of the compatible female body mods you can adjust how large you want the maximum size of most/all of the specific nodes (if you want it for guys, there is also an option for SAM body, but it’s more limited).
The setup may take a bit though, both in and out of game. In game, setting up the specific body shape may take a little bit. Out of game, it’s very simple if you use oldrim, but special edition is a little trickier due to CBBE not having a belly node (you’ll probably have to import the oldrim cbbe model into special edition for the belly node). If you just are looking for thicc thighs, then you won’t have to do anything complicated even with special edition.
As for how big you can get if you really max out some of the sliders, it’s pretty big. You won’t be a blob, but you still can get well into the obese range with right combination of sliders.
It also allows you to change how fast you gain and lose weight, it can be as subtle or extreme as you like it. You can also choose how much you want the different food types to increase weight, as well as set how fattening potions and ingredients are (if at all). Finally, there are some optional debuffs (or buffs if you set it to be that way) for being overweight/underweight. Custom food and drink from other mods can be accounted for as well, just set what category you want it to be counted as and then hit the button to register all food and drink that have been added.
You can give yourself a stamina or speed debuff. I wouldn’t recommend the stamina debuff, because of how the calculation works. Your stamina will very often snap back to normal for half a second while the game updates your weight level, this can often allow you to use power attacks in combat that you really shouldn’t be able to do. The speed debuff works just fine though.
I would say it works best if you couple it with a needs mod or if you set potions and ingredients to be very fattening if you don’t have a needs mod.
I do think it’s worth the trouble, even if you are using special edition. If you really want weight gain in Skyrim, this is pretty much the best it gets.