I have a spellcaster in my campaign that has access to true polymorph and disguise self. If, while as a humanoid, the caster casts disguise self before casting true polymorph to turn into a creature that does not have the same size and same arrangement of limbs as the illusion, would the illusion change or stay the same size and arrangement?

Disguise Self doesn't say that you can change its appearance while the spell is active, so I would suggest that changing into something that isn't supported by Disguise Self will cause the disguise to not look convincing.

The caster is turning into a huge creature. It doesn't matter to me if the disguise is convincing. What matters is if the huge form is not visible while the medium illusion is visible? I don't count this as being invisible; however, this could just look cool.

The illusion only allows you to change into something one foot taller or smaller, if the underlying creature changes to something outside of the size of the illusion then the illusion won't cover that new form.

But, the illusion hides the rest of your form. Wouldn't that effect carry over to the huge form if the illusion is cast on the medium form, hiding the huge form? This doesn't seem like it breaks the game.

What you thought was a centaur is actually a wizard who cast disguise self to look like a warrior and then polymorphed into a horse.

Quote from Primed_Horyzon >>

I have a spellcaster in my campaign that has access to true polymorph and disguise self. If, while as a humanoid, the caster casts disguise self before casting true polymorph to turn into a creature that does not have the same size and same arrangement of limbs as the illusion, would the illusion change or stay the same size and arrangement?

Since disguise self is disguising your original form, and that form got changed, I'd probably rule that the illusion no longer fools anybody

If the player wanted to have the illusion sticking uselessly out the side of their polymorphed form instead of just disappearing though, I'd probably roll with it

So, if you polymorphed into, say, a tyrannosaurus rex, the head and limbs of the caster's illusion would be popping out of the body of the beast while the beast is clearly visible. Is that what you mean, AntonSirius?

Quote from Primed_Horyzon >>

So, if you polymorphed into, say, a tyrannosaurus rex, the head and limbs of the caster's illusion would be popping out of the body of the beast while the beast is clearly visible. Is that what you mean, AntonSirius?

Yeah, something like that

One benefit of Malleable Illusion. Solves this problem.

But I agree with the concensus here. A medium size wizard casting disguise self then casting true poly into something huge is going to look like something huge with a medium sized disguise covering part of them.

As a DM, I would rule the Disguise self remains it's current size and does NOT make you invisible. 

For a creature that assumes a new form larger than the disguise self, the illusion still exists, but your own flesh prevents anyone from seeing it.

For a creature that assumes a new form smaller than the disguise, I would let the illusion work, but you are now a halfling whose real body is under a human illusion, etc. etc.

Although I agree with most of the posts responses I guess the problem stems from disguise self being able to make you appear 1 foot shorter. If you are a 6 foot human and use disguise self to make you look like you a 5 foot tall elf, some of the responses here make it seem like you would look like a 6 foot tall human with a 5 foot tall elf sticking out of the front of them. 

Quote from ThriKreenWarrior >>

Although I agree with most of the posts responses I guess the problem stems from disguise self being able to make you appear 1 foot shorter. If you are a 6 foot human and use disguise self to make you look like you a 5 foot tall elf, some of the responses here make it seem like you would look like a 6 foot tall human with a 5 foot tall elf sticking out of the front of them. 

 
No, the illusion can affect the 1 foot but the different between a Medium and a Huge creation is considerably more than that.

I agree with the consensus here. While nothing about Disguise Self says how it would interact with the caster dramatically changing their physical form; I would rule the effect of Disguise Self is limited by the size of the caster when it is caster.

That being said I could imagine homebrewing a spell that makes the caster look like something a wildly different size than they are. I think Greater Invisibility would be a good starting point for a spell like that.

Quote from Fangeye >>

I agree with the consensus here. While nothing about Disguise Self says how it would interact with the caster dramatically changing their physical form; I would rule the effect of Disguise Self is limited by the size of the caster when it is caster.

That being said I could imagine homebrewing a spell that makes the caster look like something a wildly different size than they are. I think Greater Invisibility would be a good starting point for a spell like that.

 Including the fact that the Greater version requires a 4th level spell slot instead of a 1st level spell slot.