How to change the size of the icons that appear in the Terminal.app
It looks like the arrow is actually text, printed by your shell as part of the prompt. As far as I know, Terminal is restricted to a single text size throughout the window, so you can't really resize the arrow without resizing everything else as well. You could try to comb through the font for a bigger-looking arrow.

As for the spaces, there seems to be an explicit space character after the arrow, and then another before the colour-changing sequences in your prompt. You could always remove those to make things a bit tighter.

You also have a couple of seemingly empty %{...%} constructs in there. That makes me suspect that there are actually some unprintable characters there, probably containing some terminal control codes, possibly to put some text, like a clock, somewhere else in the window. It is possible that those unprintable characters are somehow generating unwanted spaces.

It looks like you might want to read up on how the prompt generation in zsh works. This is documented in the zshmisc man page, which you can reach by running man zshmisc (for the top-level documentation, run man zsh).