Select size
When you insert a picture into a spreadsheet, it takes a snapout of the picture which is saved within the spreadsheet file itself. If you source picture is very large, i.e. it has a lot of pixels such as 4,000 x 3,000 pixels, then the size of your spreadsheet on disk is going to be large as well to save all of this information. It is more likely that when you size your picture on the spreadsheet's surface it will be much smaller that this, for example 400x300 pixels, or in most cases, a lot smaller than this.
This tab allows you to lower the number of pixels in our image by a percentage. Use the slider to set the inserted size of very large images to say 25% or 50% instead of full size.
Just keep in mind that your spreadsheet may be scaled up within Repeat Signage, so don't go overboard and resize down too far. An final image size of around double what you need is fine as it then gives you to zoom to resize or for Repeat Signage to scale up if required.
IMPORTANT NOTE - If this is an embedded image from an existing spreadsheet, rather then one inserted using Repeat Signage, if you lower the size of your picture and save your spreadsheet, then the current embedded image will be permanently replaced.