The size that fits you to the bike best. Shorter stems are better for DH, longer stems are better if you're climbing with it as well. Most people settle between 50-70mm for stem length and 760-800mm wide bars. If you're still on the small side a 730 mm bar might be a better fit, but you can always cut down a larger bar afterward.
What's your frame buddy ? Long t tubes lend theirselves to shorter stems and wider bars etc. me I'm happy with a 50mm stem and 740 bars, always have been tbh even on my DH rigs
You want to get a setup that puts you in the "right" position for your type of riding, but that is hard to do if no one knows what bike you are riding, what size that bike is, and how well if fits you in the first place. For my bike (med stumpjumper evo) I opted for a 60mm stem installed to have a 6 degree drop, coupled with a 785mm bar with a 3/4 inch rise.
In your profile you say you ride a 4 inch travel hardtail, so I hope you are talking about a completely different bike for this "enduro build."
50mm is a good place to start. Longer is better for climbing but the handling suffers a bit, which helps if you find the bike too twitchy.
As for bars, they have to fit you. My rule of thumb is when you grab the bars in front of a mirror, the inside of your hands must be where your shoulders start, if that makes any sense. If the bars are too wide, you're wasting power and making the whole thing more difficult, kind of like push ups/pull ups being harder with a wider grip. Just put your grips on wide bars without cutting the bars and toy with them until you find a sweet spot.
You want to get a setup that puts you in the "right" position for your type of riding, but that is hard to do if no one knows what bike you are riding, what size that bike is, and how well if fits you in the first place. For my bike (med stumpjumper evo) I opted for a 60mm stem installed to have a 6 degree drop, coupled with a 785mm bar with a 3/4 inch rise.
In your profile you say you ride a 4 inch travel hardtail, so I hope you are talking about a completely different bike for this "enduro build."
Hahaha of course I'm using a different bike. It's a completely custom marin vision xm8 size medium with a 150mm fox talas up front
also, where are you riding? when I lived in SoCal, I preferred wider bars. Now that I live in the PNW, a little narrower bar is better for dodging trees. I like 750.
35-50mm stem depending on whether you feel the bike needs to be shorter or not. to be honest unless your well over 6ft I dont feel there is any need for 800 or even 780mm bars. 740-750 is a good size for most people.