Yes, the Mousemen's "favourite method of attack is to silently surround their intended victim" which suggests they're sneaky, although they don't have higher-than-normal odds of surprise or thief-style Move Silently or Hide In Shadows percentages, which is the normal AD&D approach to being stealthy.
However, a Goblin doesn't have any stealth skill in AD&D either and they're really sneaky in 5E so I'm inclined to give a 5E interpretation of the Mouseman at least Stealth Proficiency if not Double Stealth.
As to size, the original text doesn't specify. They're described as stronger than regular brownies, which are Tiny and 2 feet tall in 2E with HD 1/2 (i.e. 1 to 4 hp), but a Mouseman is HD 1-1 (1 to 7 hp) the same as a Goblin.
They are a cross between a Tiny Fey brownie and a Medium Humanoid orc so could be Small, as that's the size inbetween the two, but I prefer making them Tiny as (a) it distinguishes them better from Goblins and Kobolds and (b) mice are hardly noted for their size.
Rather than literal daggers and javelins I imagine they use wee swords and spears that are so small they have the same damage as a dagger: 1d4.
The 5E
Sprite has a "Longsword" and "Shortbow" that only do 1 point of damage, so there is precedent for Tiny creatures wielding artificial weapons that do less damage than those of Medium creatures.
So maybe the Micemen have weapons that do a dice size less? Shortswords that do 1d4 slashing and Spears that do 1d4 piercing or 1d6 wielded two-handed as they're Versatile weapons)
Anyhow, I'd better stop before I'm too distracted from the Dragon Warrior conversion.