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I have a .pdf on 'Ports' on my web site, talking about some of the experiments I've done on that. I spent 'way more time than I expected to; it's a more complicated subject in some ways than you might think. But the basics are pretty simple.
Opening a 'port' will raise the pitch and output of the 'main air' resonance. This is the lowest pitched resonance that can produce useful sound, and has a lot of bearing on the low-end sound. Just what the effect will be in a given case can be hard to say: as often happens on the guitar, it's a matter of balance. More power in the low end implies a 'bassier' sound, while a higher resonant pitch favors a more 'open' or 'treble balanced' tone, so you've got opposite effects from the same action.
The further the port is from the main sound hole, and the larger it is, the greater the effect. A 2" hole just below the wide part of the upper bout may not change the sound as much as a 1" hole at the base of the neck, or an even smaller one in the tail block.
Most of whatever effect there is on the sound out in front seems to be related to the pitch change in the 'air' mode, rather than the added power output. It takes a pretty big change in power to sound even noticeably louder, but a pitch change that shifts a resonance onto or off of a played pitch can alter the sound of that note a fair amount.
The main use of a 'port' is as a monitor. The lowest notes on a guitar have wave lengths in the air that are much longer than the box, so they put those sounds out as a 'point source', about equally in all directions. As you go up in pitch the sound becomes more directional, so that the high frequencies are going out of the hole and off the top toward the audience. The player hears the low notes about as well as anybody in the room, but can only hear the higher frequencies if they're reflected back. In a big, dead, or noisy room the player doesn't have much feedback, and the effect can be pretty disconcerting. Pun intended: I've been there.
There is a fair level of high frequency sound rolling around in the box, and a port that you can see into as you play will direct some of that toward you. Again, the closer the port is to the main hole location the more the sound it puts out will be like what's coming out of the main sound hole. As with the 'main air' resonance, opening up a port alters the pitch and power of any internal resonance it hears, so different locations will produce different results.
I like to use a fairly small port, around 1" diameter more or less, just below the wide part of the upper bout on the bass side. This is large enough to be a useful monitor in most circumstances, and small enough to leave the balance pretty much where you want it. I normally cut down the size of the main hole when I'm going to put in a port, but it's a bit hard to say how much of that to do. Making it 1/4" smaller in diameter might be a good place to start. Since you already have a 'normal' size sound hole, I'd say keep the port small.
I'll note that, in the early days of using ports, some folks said they were practically 'magic', and would 'improve every instrument'. I don't believe anything is always an improvement: I like ketchup on hamburgers, and sometimes on eggs, but not on ice cream, although such things are a matter of taste. Opening a port does what it does, and it always does the same thing. That's what I spent all the time figuring out. They're useful if you play a lot of big, dead, noisy rooms ('restaurant gigs'), and folks with hearing loss seem to like them, but they're not magic. It being the 'net and all, those early posts are still out there; take them with a grain of salt.
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