How to resize quickly a imgur image?

You can specify the small version of the image by appending s to its base URL.

For example, the URL of your original image is https://i.stack.imgur.com/Brjd7.jpg.
By adding s this becomes https://i.stack.imgur.com/Brjd7s.jpg:

https://i.stack.imgur.com/Brjd7s.jpg

Some images may have an ultra-small version of the image, accessed by adding ss, thus making it https://i.stack.imgur.com/Brjd7ss.jpg, but your image does not have one.

Here is the list of all the modifiers for dimensions (not all are always available) :

s =   90×  90 = Small Square (as seen in the example above)
b =  160× 160 = Big Square 
t =  160× 160 = Small Thumbnail 
m =  320× 320 = Medium Thumbnail 
l =  640× 640 = Large Thumbnail
h = 1024×1024 = Huge Thumbnail

It may depend on the type of encoding the forum uses.

On Stack Exchange you can use the line <img src="[url]" width="[nnn]"> to specify the size quite precisely.

eg
<img src="https://i.stack.imgur.com/Brjd7.jpg" width="123">

The advantage of this method is the full size image is available to anyone who needs to see it, by right-clicking & opening in a new tab.