Brand new player
HI
A good starting point on a euph would be a Wick 5AL or 5BL (shallower cup) but it depends what reciever your instrument has. If its a small reciever you'll need a 5BS. Anouther one to consider would be a Bach 5G, but I don't recall if they do a small shank. The average euph mouthpiece (once your 'in lip') is around 26mm inside diameter, eg Wick 4AL Bach 4G.
If its a Baritone a Wick 6Bs or Bach 6 1/2A would be a better choice. But some preffer a smaller size eg Wick 9BS.
Its all very personal, a combination of what suits you, your instrument, how developed your chops are & your sound prefferences.
Good luck
Dave
Current Euphs:
York Eminence
Boosey & Hawkes Sovereign (Round Stamp/ Globe)
Boosey & Hawkes Imperial
Plus an attic of old classics in various states of repair!
Previous Euphs:
Besson Prestige (German)
Geneva Symphony
Wilson 2900 with Eminence leadpipe
Sterling Virtuoso (300 mm heavy red brass bell)
Cortios 167 II
'Gob Iron': Doug Elliott Euph 104 I 9s (plus a few others!)