Xara Cloud has 2 file limits:
100Mb file size.
1.2Gb server memory (RAM) usage.
File Size
If you try to open a file larger than 100Mb, you will see this message:
If so, it's unfortunately too large to open within Xara Cloud. You would either need to split your file, or open it within Designer Pro+ to edit.
Optimizing your file
If your file is less than 100Mb, Xara will firstly check how much memory it uses on the server. If it uses more than 1.2Gb of RAM, it can only be opened if it is optimized. You will see this prompt...
As it exceeds the memory limit, it can only be opened if it is optimized. Click Optimize to do so, or close the document to return to the file picker.
Some documents however may not exceed the memory limit, but be close to it or contain a large number of large images. If so, you may see this message...
You don't have to optimize your images, however doing so will increase performance.
What does optimizing do?
There's several things optimizing an image does, but essentially it reduces the resolution to Print Quality (300DPI). Photos from phones and stock images can be huge in size and resolution, and consequently use a lot of server memory to process.
To see a detailed explanation on what optimizing images does, please see this article.