So I am wanting to play an Atomie they are a very small fey less than 1 foot. My DMG wants to give me like a -6 str and con or something and while I understand it is up to the DM I feel that is pretty harsh. By comparison the Pixie which is 2.5 foot tall are only given a -1 str and con per the complete book of humanoids. Is there somewhere I can find stats for creatures based on size for 2nd edition or would it be based on something else. Any help is greatly appreciated.
The Dungeon Master's Guide (1993) on Creating New Player Character Races says
Creatures of tiny (T) size have a −3 modifier to Strength. Creatures of small (S) size have a −1 modifier to Strength.… All other ability modifiers are assigned by the DM. Likely candidates include minuses to Charisma and Wisdom and plus or minus adjustments to Dexterity. In all cases, bonuses and penalties should balance out. If a creature has a +1 bonus to Strength, it should have a −1 penalty to another ability. With the exception of Strength, no creature can have a modifier greater than +2 or −2 to any score. (15)
(Note that Constitution is unmentioned.) So, while Complete Book of Humanoids does, indeed, give the pixie only a −1 to Strength, that's because of the pixie's small size. The tiny atomie, on the other hand, according to the DMG, warrants a −3 to Strength (not a −6), but keep in mind that pretty much everything's at the DM's discretion when using such a race.
Humanoids, by the way, also lists maximum pixie Strength as 14. Since the typical pixie is well over twice the height of a typical atomie, a DM that rules maximum atomie Strength is 12 isn't really being unreasonable by comparison. Were a player in one of my campaigns to've insisted upon an atomie PC, a −3 to Strength but a maximum Strength of 12 seems like an acceptable compromise over a potentially crippling flat −6 to Strength, especially if ability scores were rolled in the traditional fashion.
\$\begingroup\$ Thank you very much for the information, sadly I found in the monsterous manual that they only have an 8-12 int.... wanted to play a wizard Atomie also they are night creatures... hadn't noticed that either. \$\endgroup\$– JonathanSep 23, 2016 at 19:49
\$\begingroup\$ @Jonathan I wouldn't worry about an atomie not being bright enough to be a wizard: The DMG goes on to say, "Those with penalties to Intelligence cannot be wizards. If the description in the Monstrous Manual implies that a creature is stupid, dull-witted, or in any way averse to magic and spell casting, it cannot be a priest or wizard" (16), and nothing like that's there. Likewise, orcs have an activity cycle of night, too, but adventurers seem to encounter them out and about during the day. Maybe your atomie wizard was kicked out of his tribe for his weird circadian rhythms? \$\endgroup\$ Sep 23, 2016 at 20:00
\$\begingroup\$ Thanks for all the help. Got it all sorted, going to have the -6 to str and all other issues were resolved \$\endgroup\$– JonathanSep 24, 2016 at 22:39
Based not on rules, but on physics, a humanoid a foot tall would be expected to have about 1/36 the strength of a full size person -- so instead of being able to carry a 50 lb backpack, they'd be capable of packing a pound and a half, near enough.
Compare that to D&D Strength stat, and a -6 doesn't seem unreasonable. A two and a half foot pixie would be expected, on average, to be a little more than six times as strong as your Atomie character.
Now, that said, D&D isn't very strict on physics; otherwise giants even 12 feet tall would be built more like dwarves (if they could exist at all -- a 12 foot human would weigh something like 3/4 of a ton). The best I can suggest is to work with your DM, and possibly consider playing a race for which there are available stats.