If you've selected a template where the slideshow goes the full width of the window, keep in mind that this can actually vary when viewing as screen sizes can differ greatly. The amount the photo is trimmed at the top and bottom also varies according to different screen sizes. So usually we would advise to just upload an image at least 2000px wide and then use the in-built repositioning control to determine which part of the image you want to trim off the top and bottom.
The other thing to consider is that the height of the slideshow is adjustable (by you when you are logged into your website dashboard) in the feature area settings. To access this, click the edit pencil on the slideshow and then click "FEATURE AREA DESIGN SETTINGS" in the left hand pane. Then adjust the height.
In terms of compressing the image to the right size for loading on the web, even if you upload a 10mb file (other than taking a little bit longer to upload because of the shear size of the file), the Rocketspark system will automatically compress the image down to the right size.
If you are wanting to embed text/graphics within the image for some reason rather than using the feature area text option, we would advise to give yourself plenty of trimming room at the top and bottom of the image and just make sure the image is at least 2000px wide - then use the repositioning controls built into Rocketspark.
Related Support Guides:
Editing the Feature Area
What file sizes and file formats should I use for my feature area?
Full height Feature Area images