Update: For embedding Youtube video, you can do the resizing at Youtube.com itself by either ticking one of the 4 size option (560x340 640x835 835x505 1280x745) or by clicking CUSTOM and typing in your choice of width while letting Youtube automatically fill in the value for the height to keep the aspect ration constant (see screenshot below). However the method described in this post will still be useful if you want to resize other video embed codes or similar:
Method of resizing video embed codes while keeping aspect ration constant
Irfanview which I used as a convenient tool to help resize Youtube video wouldn't work with MAC. The alternative using calculation to resize Youtube video is also a problem for some, so this post will explain how to use GIMP to help with resizing Youtube video to fit your blog sidebar or main column or whatever. (Note: Youtube video can also be resized/customized direct in Youtube website too to be covered with another post)
Open GIMP then on the menu bar, click File > New and you will get a pop up to enter the width and height of the new image. Type in the original width and height of the Youtube video (example width=640 height=505 see example Youtube embed code).
After creating the new image, go to the menu bar again and click IMAGE > SCALE IMAGE
and in the popup, type in the WIDTH a value that will fit the video into whichever part of your blog you want to embed the video into and then click on the field for HEIGHT and the value for the height to maintain the aspect ration of the video (so that it does not become distorted) will automatically appear in the field for the HEIGHT (Alternatively type in value for WIDTH then click SCALE button, the image will be scaled to new width and the value for HEIGHT will automatically be rescaled and appear in the HEIGHT field)
Now just edit the embed code with the new values for width and height (for Youtube video you will have to do it in 2 places in the embed code. Will upload an example later