NextGEN Gallery provides you with the option to automatically resize your images at upload or after you upload your images. You may want all of your images resized or only a few within a gallery.
OPTION ONE - AUTOMATICALLY RESIZE
To set the max width and height, go to Gallery > Other Options > Image Options, here you may set the max width and height after setting 'Automatically resize images after upload' to Yes, and set other options like 'Backup original images'.
OPTION TWO - MANUALLY RESIZE AFTER UPLOAD
1) Navigate to Gallery > Manage Gallery (or 'Add Gallery' window > Manage Galleries in post/page edit)
2) Select all images or individual images to edit.
3) Select under bulk actions dropdown 'Resize images' and then click 'Apply'.
4) A pop up window will appear where you set the max dimensions for the image to be resized, click 'OK' to begin the process.
Once the process is complete, be sure to click 'save changes'!