Selling full size images problem

I am a photographer and I have decided to sell images as downloads.

When I upload an image, I cant sell it at its maximum as the system tells me its too large for the size I have nominated in the price list.

File size 6600 x 4406px 300dpi
Sell size 6600 x 4405px

only way it will accepts is if I reduce it by 1px both directions.

Is there a way round this, also does it reduce the file to 72dpi or leave it at 300dpi

Please help

I am using the current download from the site yesterday Sept 6th

https://www.ads-software.com/plugins/sell-media/