I suggest you dissuade the person from trying a tiny creature. As a melee character, he will be useless. As a caster, he is going to get a ton of benefits, and annoy the GM.
Bestiary Table 3-1, p292 sets the base stats before the character does his point buy as STR 2, DEX 14, CON 8, while the mental stats at 10 base each. So, a tiny bow would be a d4 base damage, but with raising strength to a STR of 5, you are still looking at d4-3. In other words, he would be doing 1 non-lethal damage more than half the time.
Now, if you allow him to be a caster, starting with a 14 dex before the point buy, and +8 stealth modifier due to size, the sprite sized caster is going to be over optimized and unmanageable. A first level sprite sized Shadow Sorcerer starts at +14 stealth if they don't raise their DEX past the starting base. Yeah, you want to avoid this. Just say no.
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For Joah: Bestiary also on page 292. "Most Medium creatures have a
base speed of 30 feet. Quadrupeds and Large creatures increase this by 10 feet each. Smaller creatures decrease this base speed by 10 feet. If a creature is particularly fast or slow, modify the base speed by 10 feet."
So, Small or smaller bipeds move 20. Small or smaller quadrupeds move 30. The biped vs quadruped might have tripped you up in giving your assassin speed he shouldn't have had. Brownie gets 20', house cat moves 30'. The Sprite in the Bestiary III is diminutive, and only moves 15'.