Very nice image! Do you use scanned textures of is it 100% Photoshop? 'bout the brush size: No. Not like the round and square brushes that come with PS. Can't change it. BUT. You can, in a way. If you make a large original (and use a pen, Wacom or Graphire or whatever) you can vary the size by change the size setting of the brush to 'pressure'. But you need to control your pressure very precisely, if size really matters. Or else you can create your brush in different sizes and alternate by hand (by selecting one when you need it), but you probably thought of that yourself...

hi, welcome to the forum. my way would be like this...(i've not tried it, but should work) reduce the size of the picture (that should become the brush) to your desired size (eg 320x160)..save it in gif-format with transparency on the background. load it into the desired scene and and place it on the original background...you can transform it with freetransform, because it has an own layer then. my english isnt perfect, i just hope you get the point of what i mean :o)