GIMP Version: 2.10.20
Operating System: Windows
OS Version: Windows 10
GIMP Experience: New User

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When I 'Export as...', I'm not seeing any dialog box to specify size. Is there a way to export a lower resolution bitmap from my original without first resizing the original? thanks.

When you save as PNG, you should get a dialog with compression and some tick boxes. If you don't need the data you can uncheck the boxes to reduce file size and play with the compression slider. Other than that, there is a plugin called Web Export which gives a "save for web" menu. I can vouch that the 64 bit version works well in Gimp 2.10.

http://gimpchat.com/viewtopic.php?f=9&t=19092

In my opinion, for Windows a much better option (multi-parameter adjustable) is Riot 1.0.1 (32-bit plug-in for Gimp or stand-alone 64-bit application).

Portable Network Graphics (png) is a lossless format, as above comments the compression setting is 0 to 9 and Gimp already defaults to 9
No scope for reducing size there. Using a 512x512 pix RGB Lenna.png image and comparisons, all 9 compression. Some options.

Lose any un-needed transparency: with an alpha channel, file size = 544k without an alpha channel = 468K

Further size reduction involves losing colours, usually RGB -> Indexed colour mode (gif format is Indexed) Reduces quality as well, although might not matter if the image only contains a few colours.
No transparency, 256 colours dithered, indexed file size = 200K 56 colours dithered, indexed = 124 K

The successor to png seems to be webp and that image, as a lossless webp file size = 432K (png 468 K)

..but it does depend on your images so try a few experiments