On my road bike i make one cassete 8 speeds. When the chain is in the big ring and smallest cog the RD is not level with the ground as i have seen images online. My chain is be too long?

A picture is worth a thousand words! Please post a photo.

Above all your chain must be long enough to allow you to shift to the big cog/big chainring combination easily, without straining anything. Check this by GENTLY turning the cranks by hand and STOP if you feel any resistance. There should still be some travel left in the derailleur. You may not intend to use this combination but you may forget and shift into it anyway, and if the chain is too short the damage, which can be severe, will have already been done. Here is the procedure I use to size my chains: Derailer Adjustment Other methods may result in a slightly longer chain, which is preferred by some.