Sorry that you cannot change the brush size in Photoshop.
Which version of Photoshop are you using and what is your operating system?
Would you mind sharing a screenshot including the Layers Panel from Photoshop and showing the Brush settings?
If the strokes are on a Layer of their own you could add the Layer Style Stroke or Outer Glow, if not a Filter (Minimum or Maximum for example) might help depending on the background.
Please note that you should always paint your brush strokes on a separate layer, never on the background layer, that way you can delete the layer and start afresh if you make a mistake, you could step back in the history panel if you haven't closed out the document.