I just checked and other resizing options aren't working either.

I found out that when I open "edit brush settings" and setting in "predefinedd" tab in the dropdown list with tags there is nothing selected. If I choose "all" all predefined tip brushes start to work normally and can be resized.

EDIT: (correction of some random mistake in the way I constructed the sentence in last post):
I found out that when I open "edit brush settings" and look  in "predefined" tab  there is nothing selected in the dropdown list with tags. If I choose "all" from this list, suddenly all predefined tip brushes start to work normally and can be resized.

It reproduced for me yesterday; but today it does not reproduce. The difference is that I installed latest build from Lime PPA. So it seems to be fixed together with the bug which caused non-displayed brush presets in "edit brush settings" dialog.

Also does not reproduce on my desktop system with today's build from git calligra/2.9.

I got the same report from a Japanese user for Krita 2.9.5. I think this could be caused by 349026 - localized UI and "all" tag issue. Gradient and pattern menu looked like no tag is selected on launch - I guess similar thing happened with brush preset(which results no tag is selected, causing this issue).

That Japanese user who had this issue(and contacted me) confirmed upgrading to 2.9.5.2 solved it.

Awesome, closing it then (but what a !@#$% bug...)

This bug still exists in krita 2.9.6 in the same form (and is really annoying, since it forces me to take additional steps to be able to use krita normally after every launch).

Yup, i reproduced the bug also (Kubuntu 15.04, last Krita-testing build from Lime PPA). it seems the bug reproduces when I select GUI language (Settings -> Change application language) to language different from current system locale. My system's locale is ru_RU.UTF-8; if I'm using Russian GUI in Krita (default one), brush resizing works fine; but if I switch to english UI, the bug is here.

I don't see the correlation with language in ubuntu.
In my case this whole problem is strictly connected to what predefined brush tips are currently listed in the brush settings tab. When starting krita none are selected. If I manually change i t to all, the brushes start working normally. 
But if I later use in the brush setting any tag filter or the "enter resource filter here" field to change a brush tip it breaks the usability again. So basically if anything other than "all" is selected in brush settings -> predefined  the every predefined brush will not work correctly. It will change the brush tip after every resize to the first tip from the currently selected list (or not resize at all, if this is just at the start of krita, when none are selected as default).

The that makes it a duplicate :)

*** This bug has been marked as a duplicate of bug 348097 ***