We have an extension that allows the user to input a color in a dialog field of type "string"

The input box is quite wide and causes the entire dialog box to be unnecessarily wide.

Anyone know if there is a way to specify how long it should be?

No you cannot set the width in pixel for example of the string box.

You should be able to 'bookend' the text box by adding a <separator/> (or other element ) inside the same <hbox>

This should stop it growing excessively to the right.

<hbox>
<param name="name" type="string" gui-text="Input:" max-length="100">Some default text</param>
<separator/>
</hbox>
<hbox>
    <label>Hello</label><label>this is a test</label><label>hello</label>
    <label>Hello</label><label>this is a test</label><label>hello</label>
    <label>Hello</label><label>this is a test</label><label>hello</label>
</hbox>

It's also possible to limit the number of characters in the box, but is does not affect its size.

https://inkscape.gitlab.io/extensions/documentation/authors/inx-widgets.html?highlight=inx%20parameters

Thanks for this info.  (and apologies for the late thank you)