As just a general rule of thumb, you do no need to resize your images for Squarespace. Squarespace creates 7 different sizes of your image (for iPhone, iPad, different screen sizes, etc) so you do not need to resize, unless you are experiencing EXTREMELY slow load times - and that is very rare.
Here's what Squarespace has to say: “Squarespace takes the guesswork out of formatting your images for display on the web. We create seven variations of each uploaded image, to ensure perfect display on any screen size.”
As a general rule of thumb, you may want want to follow the following guidelines just in case:
- Keep your image width to 1500px maximum.
- Only use the following image types: .jpeg, .png, or .gif.
- Keep your image size less than 500 KB for best results.
See more info on this here: https://support.squarespace.com/hc/en-us/articles/206542517