subwoofer size that goes with in-wall speakers?

Unfortunately the above question suggests a lot of misunderstandings about both speakers and subwoofers.

First off, congratulations on choosing an in-wall speaker that looks pretty nice on paper. I have long had a pair of Q15s, and appreciate the theoretical advantages of coaxial speakers.

The first problem with your question is the implied idea that you can tell how much bass a subwoofer generates based on the diameter of its driver and that larger diameter drivers necessarily generate more bass.

In fact a well-implemented subwoofer does what you want it to do within useful limits. You have several effective adjustments on your AVR for controlling it.

The idea that larger subwoofers necessarily generate more bass is wrong because how much bass the subwoofer generates is up to you. I suspect that your real fear is that your subwoofer will have boomy bass, and the answer to that fear is counter-intuitive: Generally, boomy bass is due to subwoofers that lack sufficient bass extension or deep bass capability.

The important parameter for subwoofer performance is bass extension, or how deep or low the deepest or lowest frequencies in the music or movie can go and still be effectively reproduced. That is not necessarily a function of the diameter of the woofer driver. It is very possible and even not uncommon for a 8 inch driver to put out more bass than a 10 inch driver.

You see the effectively of a subwoofer driver is dependent on how much air it can move in a well-controlled and accurate manner. All things being equal a larger woofer driver can move more air, but all other things are rarely equal. One can find 8 inch drivers with linear operation over distances ranging from nust a few mm to maybe a dozen or more mm. Thus an 8 inch driver with 12 mm linear travel and outperform a 10 inch driver with 3 mm linear travel.

Another serious issue is that the bass response of open back wall-mounted speakers like the one you seem to have chosen is very dependent on how it is installed.

So, your question can't really be answered well as it is currently written.

In general, it takes a 12-inch subwoofer to give sufficient clean undistorted bass in most rooms, and at least 300 watts of RMS subwoofer power. There are a few smaller subwoofer drivers that can move enough air, but not many.

Without knowing the dimensions of your room, however, there is no way to specifically answer the question.

Ceiling height?

It is not written in the laws of physics that an in-wall speaker has to have inadequate bass. The bass you get out of a speaker has a lot to do with how you enclose it as compared to its own internal dynamics.

Do the speakers you chose have their own enclosures?

I suspect that an in-wall speaker on a wall composed of single thin sheets of drywall have a lot of bass loses and exposures to rattles in the wall.

The beauty part of subwoofers is they all fit. Big, little expensive , and cheap. Thats all they do is fill in the bottom octaves. With in walls and their inability to play bass at all, your sub may have to be placed upfront and XO kinda highish, due to the inwalls inability to do so. It will be placed upfront so you dont notice it reproducing higher frequencies as you would if it was to the side and you heard voices coming from it.

Whats your budget. Or did i miss that part....

The Ball appears to be a joke, as serious A/V subwoofers go.

The KEF model you mention appears to be pretty lightweight and limited.

If these were the only alternatives, the wall-mounted speakers would appear to have the greater potential for bass.