Foregoing the editorial comments about alleged juried art shows and their knowledge (or rather lack thereof) of digital imaging (ooops, guess I slid that in ;-) ).....
.... in LR - File>Export, choose your saving location, select JPEG as the file type, choose your quality setting, colour space as sRGB then in Image Sizing set your dimensions to 800 pixels on the long edge and resolution to 300 ppi. Click Export and you're done.