The most important measure to keep in mind when defining the right resolution to upload is DPI or Dots per inch. This could be defined as the number of individual dots that can be placed in a line within the span of 1 inch (2.54 cm).
Most printing laboratories work with DPIs that range from a minimum of 150 DPI to an optimal resolution of 300 DPI.
This means that if your biggest print is a 10x7” your photo file should (at 300 DPI) 3000 pixels on the long edge.
Uploading your photos with a resolution of 3500 pixels on the long edge should be sufficient to cover most print sizes and products.