In tmux can I resize a pane to an absolute value

usage: resize-pane [-DLRUZ] [-x width] [-y height] [-t target-pane] [adjustment]

ie.

resize-pane -t 1 -y 5

What about Ctrl-B then (pressing Ctrl) + arrow?

If in tmux < 1.8, doing this by Ctrl-B then (Esc + arrow) * n, where n is the number of times you want to resize.

In tmux 1.8, the size of the panes may be adjusted interactively. Under the default key bindings this may be done by the prefix key (C-b) followed by the meta key combined with arrow keys (M-arrow), where arrow is one of the up/down/left/right arrow keys.

E.g. Ctrl-b followed by Alt-Up will adjust the size upwards.

Note that the Alt key may be held down while the up/down/left/right keys are pressed multiple times to make multiple adjustments.

E.g. Ctrl-b followed by Alt-Up-Up-Left will adjust the size upwards by two movements and left by one movement.