Small or big tubes/valves for pre amp ?

That’s a generalization that large tubes sound better than small; it
depends on the implementation of the component. IMO, the 7308 which is a substitute for the 6922 sounds much better in terms of dynamics and realism. And most preamps use the 12AU7 due to the availability of NOS tubes, reliability, and good SQ in many different circuits.

The 6SN7, 6SN7GT, and its variants are very special; many types were ruggedized for military use in WWII, so they have excellent build quality. Their sonics vary greatly depending on the version and the manufacturer.
They have a large bottle and an octal base and may have the most pleasant and musical sonics of any preamp tube. They’re also excellent in the gain stage of amplifiers. Their greatest flaw, which is less common with small signal tubes, is that they can easily become microphonic. I admit it’s my favorite tube, but it does take more time and expense to find the right NOS due to the many different sonic signatures and also to test for low noise and microphonics.

I spoke to Don about purchasing his preamp, but it was a 3 month wait. There are many fans of his components online, not only because of the use of the 6SN7, but how it is implemented into his designs.

I have had some correspondence with Don, he seam like genuine nice person and very helpful, but of course he will not talk down his own product, and nor should he.
As I know the the flavor of many nos 6922 and they can be very different, I have no knowledge about the 6SN7 family of tubes, have heard also that they are a bit more fragile than the smaller tubes and do not last as long.
I like the Mullard sound, but with detail, am allergic to thin/shrill sound.
Tone, decay, timbre and natural voices is something I listen for in the music, I find that many amps and speakers today can replay a woman voice perfectly, sometimes spellbindingly good, but male voices not so good, often sounding neck up only, no chest!

I have my dream speakers now, and with my solid state amp I have problem with Van Morrison for example, sounding too thin, but with my hybrid amp he sounds fantastic!
Going to a tube pre into a solid state amp, a Belles or Van Alstine I am hoping to get more detail and maybe a more euphoric sound, also my hybrid amp makes me listen to music for hours at a time, my solid state amp, not so much! 

Go for it.
Use Psvane Globe 6SN7 from Grant Fidelity. You might want to ensure the pre can take their size. They are amazingly detailed.

Any 6F8G with 6SN7 adapters beats all 6SN7s IMO.
I personally like 12volt  tubes like 12AX7,12AU7,12AT7,12BH7...etc. better than 6SN7 . Like Lowrider said ,many of the  6SN7s that have good balance and Bass are microphonic.. like Ken Rad or Tung Sol.
The  vintage Sylvania 6SN7s can be  magical in all areas , except , they do NOT have good Bass. None of the Sylvania 6SN7s have good bass It is very weak . My amps which are the Granite Audio 864SR use 6SN7 in both V1 and V2.As long as I use something with good bass in V1              ( I actually use 6F8G) then using the Sylvania 6SN7-GT  in V2 works great  for me.