Pretty sure that all Quickfire keyboards have the "non-standard" key sizes for the CTRL / ALT / Windows / Fn keys. They are not the only manufacturer ever to do this as the Razer Black widow has the same arrangement and the Noppoo Choc Mini is very similar.
The typical key sizes in the CTRL / ALT / WINDOWS / SPACEBAR / etc row are (from left to right are)
1.25 1.25 1.25 spacebar 1.25 1.25 1.25 1.25
Whereas Coolermaster have gone with
1.5 1.0 1.5 spacebar 1.5 1.0 1.0 1.5
I will refer to the "lower left" cluster, meaning the CTRL, Windows, and ALT keys to the left of the spacebar.
I will refer to the "lower right" cluster, meaning the ALT, Windows, Menu/FN, and CTRL keys to the right of the spacebar.
Implications for directly replacing keycaps (aka finding replacement keys that are exactly the same size / profile):
A) The spacebar is a non-standard size because a typical lower left is 3.75 spaces wide, while the CM layout is 4.0 wide. The lower right is the same overall width on both, 5.0 spaces. Consequently the CM spacebar is 0.25 spaces narrower.
B) The CTRL and ALT keys cannot be replaced with any key that is the same size / profile. These keys are R1 1x1.5 and
The only replacement keycap available in the 1.5 width is the "TAB" key for row 3 (R3 1x1.5).
C) The Windows and Menu/FN keys can be replaced, but not with typical keys. Typical replacement keycaps will be 1x1.25 instead of 1x1. HOWEVER, because these keys are R1 1x1 they can easily be replaced since that size is commonly used for the ZXCV row!!! Most likely you will have to get custom printing but the size / profile (R1 1x1) is readily available, so replacements will feel the same.
Implications if you're willing to accept some "issues"
D) The CTRL and ALT keys CAN be replaced with typical replacements if you're willing to accept that the replacements are fractionally smaller. After all the typical key is R1 1x1.25 which is just a narrower version of R1x1.5. If you do this you end up with larger gaps around these keys.
E) Here comes the tricky part. Take a look at the spacings between the switches on the lower left. The typical spacing is 1.25 between each switch. Fortunately the CoolerMaster layout ALSO has 1.25 between each switch!! So if you replace the CTRL and ALT keys with typical 1x1.25 replacement keycaps the extra gap that you get between them lets you drop in a typical Windows replacement key sized at 1x1.25!! As is always the case with using the 1.25 wide CTRL and ALT keys you end up with wider gaps to the left of CTRL and between ALT and the spacebar, but at least you're using standard size keycaps!
TLDR: The funky sizing means no replacing the spacebar, but if you're willing to have some extra gaps or different profiles it's possible to replace most / all the keys.
Lower left solutions:
Option 1) Correct Profile but with Gaps: Use typical replacements (R1 1x1.25). Only downside is that the control key is slightly indented compared to the rest of the left side of the keyboard, and there is a gap between ALT and spacebar.
Option 2) Wrong Profile but NO GAPS: Use "TAB" key replacements (R3 1x1.5) for CTRL and ALT, then use some 1x1 key for Windows replacement. I'd suggest using R3 1x1 for Windows that way all three keys are the R3 profile so it's not so noticeable. Some people might like this better since R3 is pretty flat and R1 is angled quite a bit (sorta how some people like reversing the spacebar).
Lower right solutions:
- - - -Replace Windows and Menu/FN with 1x1 keys of your choosing. Nothing else fits because the switches are 1.0 apart so you CANNOT use wider replacements.
Option 1) Correct Profile but with lots of gaps: Same as Option 1 above, except this time you get gaps on both sides of CTRL and ALT.
Option 2) Wrong Profile but NO GAPS: Same as Option 2 above. Same advice holds that if you're with "TAB" keys for CTRL and ALT you should probably also go R3 profile for Windows and Menu/Fn so that all the keys in the cluster are the same R3 profile.