After reading all comments I opt for the "brush state is persistent, but you can discard your changes (such as size) / go back to the original".
Martin: do brushes have version information? If so, customized brushes could simply increment the minor part, whereas mypaint/author changes/updates increment the major value.

+1 for the autosave of the brush radius and other settings then , and usage of the 'reset all' button if a beginner want to go back to the original setting after hacking too much the brush.

Btw, the button label 'Reset all' is confusing for me. I never touched it, because I thought it would reset all brush. Thanks a lot, it reset only active brush. I propose 'restore to default' or something that sound less dangerous :D

I posted this in a separate bug report here : https://gna.org/bugs/?18762

Looks like I missinterpreted the purpose of the original request. Let's focus on a single problem per bugreport: "The brush does not remember size changes". Please use a different bugreport to discuss "cursor does not match stroke width".

I never had this problem myself because I usually stick to a single brush. We already do remember all brush settings together with each tool: if you restart mypaint all your modifications are preserved.

I think it is very clumsy if you have to press a "save" button every time for this. Even if the button is on the toolbar, some people will just not get it or not bother. And I think that if we remember any changes, they should be persistent. It is very silly if you have to re-do them after every start of MyPaint.

I think the most consistent thing to do is to remember all brush settings automatically in the same way that we already remember them per device. When you select the current brush (again), the real settings are restored. It already works this way with the tool settings.

The only disadvantage of this is that it will get confusing when you start to create a new brush, or just explore the brush settings. The next time you select the original brush, you will have all your experimental settings restored.

However, thanks to Andrew, we already have a GUI to show whether the brush has been modified or not, including a "reset all" button! So maybe it will not be too confusing. In addition, we could reset all temporary settings of the original brush, as soon as you save it with a new name.

I understand your POV Mihai, and here I'm also against uniformisation of brush radius and zooming. I zoom not a lot , btw.

I had very often the same issue ; working at different resolutions for various project lead me to adjust constantly the preset I use at the start of a new painting ( mainly size / opacity ... more rarely pressure curves ).

So, I became used to do Ctrl+B , then in the brush setting, press the 'save' button to 'bake' the preset. Then close the brush setting.

With the new GUI toolbar , there is now - Thanks to Andrew - a notification when the brush settings get changed. I suggested by the past to add in the dropdown panel a 'save preset' button.

I think a faster access to this save option can be a wiser solution to solve your and mine little problem . Because saving or overwriting a preset should be , imo, a user action not a autosave.

Just my point of view.

About the radius visual feedback ( brush outline ) of huge sized brush issue where the outliner is limited to a fixed radius, yes that's another problem, I agree on it.

I think what we need is a brush "state". Whenever you pickup a brush, it should remember its last state in terms of every possible settings excluding color. If the user wants the default settings (the one that gets loaded now every time you switch back to a brush), there should be a shortcut to do so. I think most of my dead time I spend while painting is re sizing brushes when I switch back to them.

And yes, there should be a way to determine approximately how big a stroke really is, as the circle doesn't do that and it's frustrating.

Typical scenario:
I pickup the Airbrush tool (from Ramon's set), give it a stroke and notice that its way bigger than the circle shows (second problem I've mentioned).

Then after I re-size it to what I want, I do some painting with it and notice that I need to correct some small detail somewhere. So I pick up another brush, correct the detail, then click on Airbrush again. Now it is as big as it was the first time, so I have to resize it again. Quite painful.

So keeping each brush's state until an explicit reset to default action is done by the user is the way to go in my opinion.

A "ink" brush and the "bulk fill" brush are supposed to have a different radius. I don't think this is a bug.

However it is a problem that you can't change the overall resolution of all brushes. Because of this, brush creators who tend to work at high resolution will make bigger default radius than brush creators who prefer lower resoultion.

(Didn't we have a bug about this already.)

It will be nice to have a option to uniformize the brush radius size for all the brush. Currently the radius changes when we change the brush.