Hi @vyvo!
It really depends on what you want to achieve. If you are fine with smoothed fields as I previously mentioned you can use RANS and a corresponding turbulence model. If you are interested in “instantaneous snapshots” of your flow LES is the way to go. With “length” I mean the longest direction of the cylinder in this case. You can either perform a “2D” simulation but if you are interested in 3D structures you can go with the model you have that’s fine.
If you think 2-3D and 6-8D is sufficient you can give that a try and see if the boundary conditions affect your solution 
For y^+ I can give you two nice resources @Get_Barried and I have put together:
I am also inviting @DaleKramer here who is already an expert I dare say 
Best,
Jousef