from polymorph via srd
White Dragons by Age
Age Size Hit Dice (hp) Str Dex Con Int Wis Cha Base Attack/ Grapple Attack Fort Ref Will Breath(DC) Frightful Presence DC
Wyrmling T 3d12+3 (22) 11 10 13 6 11 6 +3/-5 +5 +4 +3 +3 1d6 (12)
Very young S 6d12+6 (45) 13 10 13 6 11 6 +6/+3 +8 +6 +5 +5 2d6 (14)
Young M 9d12+18 (76) 15 10 15 6 11 6 +9/+11 +11 +8 +6 +6 3d6 (16)
Juvenile M 12d12+24 (102) 17 10 15 8 11 8 +12/+15 +15 +10 +8 +8 4d6 (18)
Young adult L 15d12+45 (142) 19 10 17 8 11 10 +15/+23 +18 +12 +9 +9 5d6 (20) 17
Thematically you fancy being a Young Adult White? go for it.
now, if you wanted to grow your own dimension of dragons that would be spiffy and thematic.
but for actually 'being' a 'dragon', i would leave polymorph alone and maybe consider just mind switching one.
i think personally, i would just use an illusion to look like a dragon.
iirc, a 'dragon polymorph' spell exists, but i stand by my argument. just look the part.