How to Resize a brush as you paint?

I think you're looking for is a tapered brush stoke.  See link below.

How to make a tapered brush stroke? .

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If you want a resutl as shown below, set your size jitter to fade and enter your length.

While you are painting, as JJMack said, you can't. But for more strokes, you can use the bracket keys next to the P on your keyboard [ ]

You can not with a mouse with a normal mouse device diver.  You need to get yourself a digital Pen.   If you want to use a mouse search for a mouse device diver that simulate pen pressure. Perhaps one exists. You can create Paths and stroke  a path with a brush and simulate pen pressure.  The may work if all to need is a few strokes.  You not doing to do a head of hair that way.  You can also use sample brush tips with a mouse. for things like hair, grass etc. Photoshop brush engine is quite flexible. Still a Pen is a better device to use with a brush tool.