Fuel Injector Questions

First off, I have an '87 Mustang with a stock 5.0. Speed Density. I do have a nitrous kit installed on the car.

I have a fuel leak on the back side of my motor that appears to be coming from one of the fuel injectors from what I can see. I have a few questions regarding this.

1.) What size injectors are in the car from the factory?
2.) I would like to put bigger injectors in if I have to replace them. I eventually plan on upgrading the car to Mass Air and put better heads and a better intake on it, but not right away. Can I upgrade my fuel injectors now and will it run normal even with the speed density setup it has now? If not, I will just have to put the stock size injectors back in and upgrade them later.
Are there any common leak points other then the injector that I should be looking at? It is kind of a hard area to see and it is wet with fuel all over back there now.

Thanks!

Update on questions...

Thanks for the replies guys.
I just finally got home from work and took a closer look at my fuel leak. I was only able to take about a 1 minute look at it on my way to work today when I noticed it was leaking so I wasn't sure what the deal was until now.

It is leaking fuel from the fuel pressure regulator, not the injector on the back like I originally thought. It looks like it will still be a pain to replace though just like the injector would have been.

Will I have to remove the upper intake to replace the regulator and what can I expect to pay for a new one?
Is this a pretty common leak point?
Thanks!

Regarding the adjustible regulator

I would love to throw in an adjustible regulator, but the problem is, I installed the Edelbrock "Dry Type" nitrous kit for '87 - '95 5.0 Mustangs. Problem is, that nitrous kit comes with a fuel pressure safety cutoff switch that I had to mount to the test port for the fuel pressure on the fuel rail. I don't really have any way of testing my fuel pressure since I did that. You have to take out the port and install this different port in its place and hook the switch onto that with some teflon paste.

I'll have to take a closer look at that regulator I guess and see if I can tighten it down.