the brush for selecting is too narrow

I think you are talking about Generative Fill. You can make the brush size bigger by clicking on settings and sliding the brush size slider over to 100%. 

Also, if you zoom out (two finger squeeze motion on a Mac), the brush size effectively gets larger (the brush size remains constant as the image size changes), as you can see here...

Normal (the original selection circle is hard to see so I enhanced it in this image):

Zoomed out:

If you were referring to something other than Generative Fill, please let us know.