How do I custom size a photo? Like the largest photo I've done has been an 8x10. I want to do a 16x10 - so in the "aspect ratio" part, do I put in "16x20" or is there something else I use for the ratio? Like when I click on "custom" it automatically comes up 1.000x1.000 -- please help!
Here you want to make sure your image has the right aspect ratio, not real size. Specify 16x10 in "custom".
When exporting, you tell LR how many pixels the exported image should have. This can be done by either specifying pixel dimensions directly or dimension in inches/cm in combination with ppi in Image Sizing.
I'd suggest to enter 16"x10" in Image Sizing and choose the ppi your printer (or printing service) uses.