no problem!

Yeah, you can just change the number values. First number is width and second is height (in pixels).

For tumblr post settings 268 is the best width for 2 by X sets and like this one (268 x 350). If you want to capture the entire video you want to have a 540 x 300 size like this. I personally don't always do this since I don't think the long rectangular gifs look too good when there are less gifs so I usually d 540 x 350 like this. It really depends on the gifs and how I want the post to look though.

But remember these are just tumblr post size recommendations. Also tumblr gif size limit is 10mb (at least last time I checked). You can make the gif larger if you want to post elsewhere (ex. twitter gif size limit is 15mb).