Resizing
Resizing can reduce or increase the number of pixels in the image (resampling the image). This reduces or increases the file size both in display and when saved as a file.
You can specify the image width/height you want in terms of pixels, percentage (of the original image) or even in inches and centimeter. If your aspect ratio checkbox is checked on, you need to specify only one of the width or height. The other one is adjusted automatically.
Resolution is the number of pixels printed per inch. More pixels per inch (ppi), will create smaller printed pixels and a smaller printed image. Fewer ppi, will create larger printed pixels and a larger printed image. Resizing can also change the image’s resolution, leaving the number of pixels in an image (and the file size) unchanged.
Resize Algorithms are different algorithm which can be used to resize the image. Fast resizing chooses an image-scaling algorithm that gives higher priority to scaling speed than smoothness of the scaled image and the resized image is somewhat jagged. Smooth  resizing chooses an image-scaling algorithm that gives higher priority to image smoothness than scaling speed. But it may make an already smooth image too smooth. Area Average resizing as the name suggest, uses the Area Averaging image scaling algorithm that takes into consideration the neighboring pixel values and their weightage. In Replicate resizing the pixels in the source image are sampled to produce pixels for an image of the specified size by replicating rows and columns of pixels to scale up or omitting rows and columns of pixels to scale down.
Maintain Aspect Ration:
To constrain the new image dimensions to the image’s current proportions, mark the Lock Aspect Ratio check box. The aspect (width-to-height) ratio defaults to the ratio of the active image. Changing the aspect ratio distorts the image by stretching or contracting it in one dimension more than the other.

For best results: