Belariath

While my character was stunned, the player of Thunder (my opponent) graciously informed me that I was able to evade while under the effects of Loathsome Screech.

So I looked around and saw this wording on Stun Bolt.
"While Stunned, the victim may not commit any offensive actions, but may still defend as normal, including if they wish to activate defensive spells, evade or stumble away."

Hence, I started to wonder. Does this wording also apply to all spells that "Stun" ?
(eg Loathsome Screech, Fire Ball, Flame Thrower etc)

Since some of my other characters also have them and I should also let those subject to "Stun" know about this.

Pretty much, there may be stun that states its effects as something specifically, completely different, but to my knowledge stun = unable to attack but do pretty much anything else.

Though remember you are stunned, ICly your character may be a little 'out of it' and kinda woozy or similar. That's RP-wise however, mechanically you are just unable to attack.

correct!

That's great to know.

Some spells have a certain radius, typically 2 round cast spells like

Blizzard (radius 30' from caster)
Fireball (radius 30', centred up to 5'x15 slots from caster)
Chain Lightning (no distance indicated, presumably any distance as long as there is line of effect)
Epicentre (no distance indicated, but does not affect flyers)
And probably some other spells.

Assuming one flees away as quickly as possible on foot while stunned, is there any expected distance to cover in one round?

Note the stun wording in Stun Bolt says "stumble away". Does this reduce the distance moving away, and does it imply a stunned character can never out run a character that is not stunned and chasing it?

nah once you start getting to the larger numbers like that there's no considerable amount of distance you could cover in one turn if you were already engaged in combat with your foe. It's more or less there so people can't start using these spells to do really far off things. Rule of thumb is if you are in combat with said person, they should be able to hit you. Of course you also have to use common sense, if someone is up in a tree obviously far away pelting you with arrows and you have like a 2 yard radius for a spell, yer not going to him 'em.