By default, dslrBooth sets your template for optimal printing on 4x6 paper at 300dpi. If you have a high resolution printer than can print 600x600dpi, go to you printer settings and change the print resolution to 600x600.
Here's how to do this on DNP printers. Your printer settings might be slightly different.
Then go to Branding, Template Details, Change paper width and height to double of what they are.