Resizing panes in tmux can be quite difficult in default tmux, I use a set of keybingings to help resize panes in the rare occasions that I do need just a bit more space. I set the keybinding to the same as my split navigation bindings but shifted. They are very vim like (h,j,k,l).

# resize panes
bind -n M-H resize-pane -L 2
bind -n M-L resize-pane -R 2
bind -n M-K resize-pane -U 2
bind -n M-J resize-pane -D 2

Most often when I need to resize panes I just grab the edge of the pane with my mouse. Yes the mouse, its not that often that I actually need to change the size of a pane.

# Enable mouse control (clickable windows, panes, resizable panes)
set -g mouse on

for more information on how I navigate tmux, check out this full post