AoE and occupied spaces

[VertDeLion]

Quick question about AoE spells that have a point of impact, such as Fireball.
Say there's a colossal creature taking up 40ft of space, in its entirety. Right against it are allied melee players. There is a wizard wanting to cast an AoE spell on the colossal creature (for this example, lets just say Fireball)
"A bright streak flashes from your pointing finger to a point you choose within range and then blossoms with a low roar into an explosion of flame"
Could - using a grid map - a wizard choose the center tile of the colossus so only it takes the fireball? or since the colossus takes up the entire tile space, the spell could hit no further than the outer squares, thus hitting the ally melee fighters too?

[hymer]

Quick question about AoE spells that have a point of impact, such as Fireball.
Say there's a colossal creature taking up 40ft of space, in its entirety. Right against it are allied melee players. There is a wizard wanting to cast an AoE spell on the colossal creature (for this example, lets just say Fireball)
"A bright streak flashes from your pointing finger to a point you choose within range and then blossoms with a low roar into an explosion of flame"
Could - using a grid map - a wizard choose the center tile of the colossus so only it takes the fireball? or since the colossus takes up the entire tile space, the spell could hit no further than the outer squares, thus hitting the ally melee fighters too?

By the naked rules, you can shoot through creatures' squares, and there is no size limitation on that. So there's nothing keeping the caster from hitting only your colossus - nothing preventing the magic getting to where you want it.

[Ninja_Prawn]

I guess it's DM's call? I mean, you can't target most of those spells at a point you don't have line-of-sight to (and in the case of Fireball, you probably need line-of-effect, too), but I feel there is ambiguity as to whether a gargantuan monster physically fills its squares, or just nebulously occupies them, and whether either of those situations blocks line-of-sight/effect. I feel (as a DM) like I might need to rule on a case-by-case basis for this one. I mean, if it's a giant, you can probably shoot through its legs or something, but if you're attacking something solid like a fortification, maybe the Fireball should explode on the outside of it.

In most cases I imagine it would be possible to aim the spell so as to avoid friendly fire (maybe by aiming above their heads?), but I would reserve the right to say otherwise in specific instances.

To be fair though, the only cases I can think of where I'd have the target block line-of-sight are where the target isn't a creature.

[Cespenar]

If it's a creature, it doesn't block line of sight, and you can select any point you want for Fireball.

[LordVonDerp]

Quick question about AoE spells that have a point of impact, such as Fireball.
Say there's a colossal creature taking up 40ft of space, in its entirety.

Could - using a grid map - a wizard choose the center tile of the colossus so only it takes the fireball? or since the colossus takes up the entire tile space, the spell could hit no further than the outer squares, thus hitting the ally melee fighters too?

Two things to remember
1) Creatures don't take up the entirety of their space/s
2) Th ink three dimensionally, the wizard can cast at a high enough point to avoid their allies.

[VertDeLion]

Thank you everyone for your hasty replies :3