Try other forums, this for modules, not mods.
What I would try:
1. Find in polymorph.2da what polymorph you are using (there are 4 dragons), jot down their appearance numbers
2. Open appearence.2da find their appearances, copy them to new lines (new appearance number). Edit your new appearences in the colmn that say "size type". Possible sizes are:
2DA V2.0
LABEL ACATTACKMOD STRREF
0 INVALID **** ****
1 TINY 2 ****
2 SMALL 1 ****
3 MEDIUM 0 ****
4 LARGE -1 ****
5 HUGE -2 ****
3. set the right sizes
4. in polymorph.2da write your new numbers of appearances (those with new sizes)
5. save both 2das, but make sure you have a copy of them beforehand
6. put both 2das in your override folder
Result should be that your new polymorphs will be of lesser size. It may be necessary to edit other lines in appearence, but that is beyond me unfortuantely :/
Override will not work in case of modules with custom appearance or polymorph haks.