Is this a 3/8" inside diameter valve or 3/8" outside diameter? i have a brass valves that is 5/8" od. thanks...

I understand your question because I had the same one. I ordered anyway and indeed the outer diameter is 5/8 while the inner diameter is 3/8. I'm not sure what the proper terminology is or why it only mentions 3/8 but the outside of this brass valve will thread into a 5/8 fitting and has a 3/8 internal female fitting.

Will this product work for an above ground pool?

I have no idea if this will work for an above ground pool as our pool is in the ground and it works great.

Do you have a 1/2 instead of 3/8 on float

No. I have customers that buy a reducer. https://www.amazon.com/NIGO-Brass-Fitting-Bushing-Female/dp/B07F9PKMCJ/ref=sr_1_13?crid=585LVYPZBR5P&keywords=3%2F8+to+1%2F2+reducer+brass

Are you supposed to turn both knobs to adjust. I turned them and the whole thing came out of the arm?

First put adjustment screw back in. It kind of goes in at a angle. The bottom one is a locking nut you just need to loosen it. Then turn the top one up or down. After desired water level then tighten locking nut.

Brand	Float-Tec
Model Number	T26B
Product Dimensions	15.24 x 7.62 x 7.62 cm; 141 g
Material	真鍮
Are Batteries Included	No
Item Weight	141 g

Float-Tec Letro Pentair T26 EZ調整真鍮フロートバルブ 3/8インチ NPT オススレッド 3インチ アームオートフィル、給水レベラー、プール、スパ、水タンク、給水器フィット6インチキャニスター。

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Float-Tec Letro Pentair T26 EZ調整真鍮フロートバルブ 3/8インチ NPT オススレッド 3インチアームオートフィル、給水レベラー、プール、スパ、水タンク、給水器フィット6インチキャニスター。