Filter Toolbar resize?

I assume that, by "more filter expressions showing", you're referring to the expressions that show on the filter toolbar to the right of the filter text box, the "Expressions..." button, and the "+" button.

If so, then adding new expressions with the "+" button shortens the filter text box to make room for the new expression's name up to a certain point, but there appears to be a point where it doesn't show any more in the toolbar, it just shows a ">>" that, when clicked, pops up a menu with the additional filters.

That's the only current option. If you want to suggest that either 1) the filter text box be resizable or 2) the minimum filter text box size be smaller, you could file a request for enhancement on the Wireshark Bugzilla, which is the best place to request new features or changes to existing features (or to report bugs) - it's where we expect them to be filed, rather than on the Q&A site, as they can be tracked better there.