You can use the brush tool and presets in many different ways. The tools in the toolbar such as the brush tool or clone tool etc can be set to 50 x 50 for very subtle artwork or fine lines but you can also use any of the presets to 1000 x 1000 and beyond to create all kinds of textured effects / big sale changes to any image. You can extend the strokes well beyond their defined size but there are limits and the results will become fairly coarse the greater you go. Some of the tools, it probably hardly matters, but with some you will notice the coarseness of the results.
You can set the size via the preset picker. Go to the edit field marked size and then edit the value by entering 200px or 500px etc into the edit field. I always find that the easiest and the most accurate way of entering the size. If you use the slider then the value jumps quite a lot from 201px to 220px etc and it is even worse with the larger sizes of stroke (it jumps from 1760px to 2120px etc). If you are focussed on the edit field then you can also use the up and down arrows on the keyboard to go up and down to set the size of the diameter of the primary stroke / dab. There is a little restore button at the side of the edit field so you can restore the size to the original setting in a second if it goes too high. It is the same value as the value displayed in the panel beside the dab preset thumbnail. You can set the size of the thumbnail to be large etc but this does not modify the actual size of the original defined stroke. If you try and set the size to greater than 5000 then the app will display an error message (one thing PS does love, showing endless error messages !) You can also set the size in other ways such as holding down the option and ctrl key and drag on your mac (different on the pc with ctrl and alt) and drag left to right etc to increase and decrease the size. You can also use the [ and ] keys to do the re-size but it jumps 100px to 200px etc
The latest brushes size limit for CC is 5000 x 5000. If you try and create a stroke larger than that, you will see the define brush command disabled. The size depends on the version for Photoshop and with the earlier versions, the limit was a lot lower. Not sure why it has been set to 5000 as I am certain many users would love to use strokes of 10000 and beyond now (not sure how well that would go on my machine though). The size limit the other way, well when you get down to 1px then you would think that most of the strokes would be the same but a 1px stroke with a long thin preset is very different to the a very singular square preset.
You can generally set the size of the brush to the maximum 5000 x 5000 even if the original is 500 x 500. Some dabs work better than others in Adobe ® Photoshop ® when you scale them. If I decide to use a small brush stroke at a much larger size then I generally take the original source stroke and scale that up and add an effect (such as blur or poster edge effect or add noise or clumped grain - I am certain everyone has their own favourites ways of scaling though there are also many third party plugin tools that can be used to scale an image) and then re-define the stroke and use that instead of the original. The one below was originally 400px and scaled to five times the original and then grain was added via the filter gallery (clumped option) to make the cow look a little more pleasing as a stroke.
Some work better with different techniques. Actually the best technique is probably to keep the original source material as a vector and then you can import the vector EPS files as 5000 x 5000 and then define those as a stroke. The one below of a female themed artwork is not a vector though and the grain adds a special touch to the stroke but the original clear face has been lost.
Or you can always add the powerful oil filter to result to smear the result so any imperfections can hardly be seen. You don't have to use it just once, you can use it multiple times and perhaps combine with the fade to add the oil effect using blending modes as well
Or use the tools in camera raw such as the dehaze
Or use the blur tools in the blur gallery to cover the rescaling of the stroke
Or just go very abstract with the oil paint filter and other effects
You can find more about this via our video tutorial on the subject of the strokes and the sizing via the square brackets / up and down arrows etc as well as ctrl and option [Youtube] PS tool sizing video