Just to clarify
I want to take a relaxing, long bath. - Correct
I want to take a relaxing long bath - Wrong
Why is it wrong without the comma?
That's not an easy question to answer. Native intuition makes all the difference here.
It is possible to use that order without the comma, but only if you give emphatic stress to "relaxing." This could be written in a few different ways:
italics: I want to take a relaxing long bath.
bolding: I want to take a relaxing long bath.
caps: I want to take a RELAXING long bath.
But it would be wrong to use that order without that special emphasis on "relaxing" and without a comma between "relaxing" and "long":
(i) [strike]
I want to take a relaxing long bath.[/strike]
(ii)
I want to take a relaxing, long bath.
If you say "I want to take a RELAXING long bath," you mean that you want to take a long bath that is relaxing: a [relaxing [long bath]]. You do not want to take a long bath that is not relaxing. "Relaxing" restrictively modifies the unit "long bath."
If you say "I want to take a relaxing, long bath" or "I want to take a long, relaxing bath," you mean that you want to take a bath that is both relaxing and long: a [relaxing] [long] bath / a [long] [relaxing] bath. Each adjective modifies "bath" separately.
Without the comma, and with no special emphasis, only the order "a long relaxing bath" works. While it is true that if you emphasized "long" ("I want to take a LONG relaxing bath"), "long" would restrictively modify "relaxing bath," that meaning is not present without that special emphasis.
Essentially, "long" has a different semantic status from "relaxing" in the normal order (as pronounced without special emphasis): "I want to take a long relaxing bath." According to one book, "long" would be categorized as an Epithet and "relaxing" as a Descriptor. Epithets precede Descriptors.
I don't expect you to understand that, especially with the jargon terms that I haven't defined. The definitions are intricate. It is worthwhile to observe, however, that the position of "relaxing" doesn't have to do with its length. The same order would need to hold if "relaxing" were changed to "hot":
I want to take a long hot bath.
[strike]
I want to take a hot long bath.[/strike]
It is also worthwhile to observe that "long" is gradable in both cases, whereas "relaxing" and "hot" are not, unless they are emphasized or preceded by a comma. Gradability is one of the characteristics of "Epithets" as opposed to "Descriptors."
I want to take a very long relaxing bath.
[strike]I want to take a long very relaxing bath.[/strike]
I want to take a very long hot bath.
[strike]
I want to take a long very hot bath.[/strike]