BTW the game designer gave the fiendish black pudding n intelligence score of 3.
Heres my try on different size oozes (through splitting):
Assumptions:
1. First I assume that splitting an black pudding halves the HD and lowers the size category by one.
2. For size reduction effects, I reverse the rules for size increase (DMG p.12).
3. I consider the puddings feats to be a part of the creature as oozes don't get feats or skills per HD (DMG p.13).
4. It's slam damage bonus is 1.5 times Str bonus (DMG p.7)
5. Oozes have bonus hit points depending on their size per DMG p. 6
6. Damage dice reduction as per DMG p.13
7. As it's not clear how many HD a halved 5HD ooze should have, i rounded up. The ooze looses a lot of hp when dropping in size, so these "bonus" HD just make up for it.
"normal" Black Pudding:
- Huge, Face/Reach 5ft*20ft/10ft, HD 10d10+60 (+40 Con, +20 size), hp 115, base attack +7, Str 17, Dex 1, Con 19, AC 3 (-5 Dex, -2 size), slam +8 (+7 base -2 size +3 Str), damage 2d6+4 slam + 2d6 acid, Fort +7, Ref -2 Will -2
- Large, Face/Reach 5ft*10ft/10ft, HD 5d10+25 (+10 Con, +15 size), hp 52, base attack +3, Str 9, Dex 3, Con 14, AC 5 (-4 Dex, -1 size), slam +1 (+3 base -1 size -1 Str), damage 1d8-1 slam + 1d8 acid, Fort +4, Ref -3 Will -4
- Medium, Face/Reach 5ft*5ft/5ft, HD 3d10+10 (+0 Con, +10 size), hp 26, base attack +2, Str 1, Dex 5, Con 10, AC 7 (-3 Dex), slam -3 (+2 base -5 Str), damage 1 slam + 1d6 acid, Fort +1, Ref -2 Will -4
- Small, Face/Reach 5ft*5ft/5ft, HD 2d10+3 (-2 Con, +5 size), hp 14, base attack +1, Str 1, Dex 7, Con 8, AC 9 (-2 Dex, +1 size), slam -3 (+1 base -5 Str +1 size), damage 1 slam + 1d4 acid, Fort -1, Ref -2 Will -5
- Tiny, Face/Reach 0.5ft*0.5ft/5ft, HD 1d10-1 (-1 Con), hp 4, base attack +0, Str 1, Dex 9, Con 8, AC 11 (-1 Dex, +2 size), acid -3 (+0 base -5 Str +2 size), damage 1d3 acid, Fort -1, Ref -1 Will -5
All sizes retain their normal black pudding abilities and immunities.