The keybindings that I use the most when working with windows in Doom Emacs are:
- SPC w vand- SPC w sto split a window vertically and horizontally
- SPC w wto move to the next window
- SPC w h/j/k/lto move to a window in a specific direction
- SPC w H/J/K/Lto move a window in a specific direction
- SPC w ato delete the other windows
- SPC w qto close the current window
Resizing the windows is not as smooth. My usual workflow consisted in just closing windows and/or using SPC w = to rebalance the space between the open ones.
This solution was suboptimal, and after months of dissatisfaction I decided to check the other available commands and stop being so lazy.
The first solution I found was using SPC w | and SPC w _ to set a window’s width and height respectively. On their own, these commands are not very useful as they arbitrarily expand the current window, but vim powers come to the rescue!
We can just do something like 80 SPC w | to resize the current window to 80 columns width.
We can still improve the workflow a little more. Sometimes guessing the correct size is not easy, and resizing windows with something like SPC w > or SPC w + is annoying because the whole key sequence has to be repeated for each step.
Luckily we can use Doom’s hydra module. 
We need to add it to init.el :ui section
:ui
...
hydra
...
Then define this function (for example in config.el):
(defhydra doom-window-resize-hydra (:hint nil)
  "
             _k_ increase height
_h_ decrease width    _l_ increase width
             _j_ decrease height
"
  ("h" evil-window-decrease-width)
  ("j" evil-window-increase-height)
  ("k" evil-window-decrease-height)
  ("l" evil-window-increase-width)
  ("q" nil))
The last step is binding this function to a key sequence:
(map!
    (:prefix "w"
      :desc "Hydra resize" :n "SPC" #'doom-window-resize-hydra/body))
Now we can just press SPC w SPC and then hold one of h, j, k or l to resize the current window.