24x48 alone is meaningless without a metric for the pixels per inch. 24 inches at 100PPI vs. 24 inches at 300PPI is a HUGE difference. You need to know how many pixels per inch are required. Then just do the math and ask LR to give you those pixels. For example, 24 inches at 200PPI (which should be more than enough) would be 4800 pixels.
Author “Color Management for Photographers" & "Photoshop CC Color Management/pluralsight"
The easiest solution is to simply export the image without reazing it. Send the banner printer the full size image and let them figure it out. Unless you cropped the image to 1x2 proportions before exporting, the image will end up cropped when printed.
The real answer depends on some details that are missing, but a starting point is to set up the Image Sizing section of the Export dialog box as shown below:
Resze to Fit: Enable this.
Width and Height: Set the unit of measure for the W: 48 and H: 24 values. I assumed inches because you didn’t say; set it to whatever unit you are actually using.
Resolution: The banner printer’s ppi requirement was not stated, so I plugged in 150 ppi. Replace that the resolution value that your banner printer actually requires. Although you may hear that resolution should be 300 ppi for print, that’s for prints viewed up close like fine art prints. Banners often use a lower ppi value because they are viewed from a distance.