I don't see "mouse" listed in the MM, but here's a toad and a rat, and from these I'll do a mouse:
Toad
Diminutive Animal
Hit Dice: 1/4 d8 (1 hp)
Initiative: +1 (Dex)
Speed: 5 ft.
AC: 15 (+4 size, +1 Dex)
Attacks: -
Damage:
Face/Reach: 1 ft. by 1 ft./0 ft.
Special Attacks: -
Special Qualities: -
Saves: Fort +2, Ref +3, Will +2
Abilities: Str 1, Dex 12, Con 11, Int 1, Wis 14, Cha 4
Skills: Hide +21, Listen +5, Spot +5
Feats: -
<The rest really does not apply at all to a PC>
Rat
Tiny Animal
Hit Dice: 1/4 d8 (1 hp)
Initiative: +2 (Dex)
Speed: 15 ft., climb 15 ft.
AC: 14 (+2 size, +2 Dex)
Attacks: Bite +4 melee
Damage: Bite 1d3-4
Face/Reach: 2 1/2 ft. by 2 1/2 ft./0 ft.
Special Qualities: Scent
Saves: Fort +2, Ref +4, Will +1
Abilities: Str 2, Dex 15, Con 10, Int 2, Wis 12, Cha 2
Skills: Balance +10, Climb +12, Hide +18, Move Silently +10
Feats Weapon FIness - Bite -
From these two and from AWAKEN, I come up with the following (I think you might want to modify the mouse to be more like "Redwall" or "Stuart Little" so that it could use items, but that's up to you):
Mouse PC
Diminutive Animal (smaller than a rat, but not smaller than a toad)
Hit die by class as normal (Ignoring it's hit die as it is 1 or less, as normal for monster PCs. Note that I did not include the bonus 2HD from awaken - since this animal will be a PC, that did not seem appropriate. You could add them in and still be ECL 0 considering the size and net negative modifers to his stats.)
Initiative - Normal (adjusted by Dex, etc.)
Speed 15ft., climb 15ft. (Same as the rat - it could possibly be 10 if you think mice should be slower than rats))
AC Base 14 (+4 Size)
Attacks By animal HD for the bite (+ Dex bonus from weapon finesse), normal by class otherwise
Damage : Bite 1d3-5 or by weapon ("-5" to make it weaker than a rat bite)
Face/Reach: 1 ft. by 1 ft./0 ft.
Special Qualities: Scent
Saves: No racial bonuses
Note: Ability score adjustments are found by taking 10 from the ability score in the MM. I adjusted the MM score by AWAKEN which gives average intelligence and +1 cha (well, +1.5 average, round down)
Abilities: -8 Str; +5 Dex; +0 Con, +0 Int, +2 Wis, -7 Cha
Note: Skiils are as per Rat with ability bonuses/penalties backed out of them
Skills (Racial Bonuses): Balance +8, Climb +16, Hide +8, Move Silently +8
Racial Feat: Weapon Finesse - Bite
This looks to me like an ECL 0 character. Stats come out at a net of -3 physical and -5 mental, making up for any other class benefits.
It's not "official," but it's as close as I could come.