NextGEN Gallery provides you with the option to 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.
BEFORE UPLOAD - SET MAX WIDTH AND MAX HEIGHT
To set the max width and height, go to Gallery >> Other Options > Image Options, here you may set the max width and height and set other options like 'Automatically resize images after upload' and 'Backup original images'.
RESIZE YOUR IMAGES AFTER UPLOAD
1) Go to Gallery >> Manage Gallery
2) Select all images or individual images to edit
3) Select under bulk actions drop-down 'Resize Images'
4) Click 'Apply'
Once the process is complete, be sure to click 'save changes'!