I just replaced all the guts of my older toilet including a new spud washer/gasket. When I put the tank back on and fill it with water there is a streaming leak coming from this gasket. I was confused because initially I thought it would only do that when I flushed, but it drips constantly.

Any ideas?

When you say "spud" are you talking about a WC with a wall hung tank and 2" chrome closet 90 -- Or are you talking about a direct tank to bowl bolted tank with a Douglas valve?

I am talking about a direct tank to bowl bolted tank. I'm not sure what a douglas valve is.

A douglas valve is simply the "flush" valve allows water into the bowl. Could it be leaking around the bolts that hold the tank on?

You are going to have to remove the tank and bolts. If there is not crack or debris around the hole in the tank I would just get a new toilet gasket. Replace the gaskets on the bolts too.

I replaced the bolts and the rubber gasket that the tank rests on. Is it possible that this gasket is not the right size? It is definitely not leaking from the bolts because when I check with a flashlight I can see the stream coming from the rubber gasket that is attached to the douglas valve.

Wait, I thought your larger gasket was leaking, not the fill valve. If the fill valve is leaking you may have skipped or reversed some of the gaskets. You should check the directions that came with valve. It does need to be quite snug. You do not need to remove the tank for this repair.

The leak is not from the fill valve. The leak is not from the tank to bowl bolts (although initially I thought it was).

After I replaced everything (including the rubber gasket that creates a seal with the flush valve) and turned the water back on, I noticed a stream of water that trickled from under this rubber gasket. There is now a built up poll of water between the tank and the bowl and it drips off the left side of the bowl.

I'm thinking there might be a problem with the size of the rubber gasket and the seal might broken somewhere. I just can't figure out where or why (because everything is brand new).

It may be brand new but disassemble it anyway. Reinspect all gaskets and washers and check the tank opening for a crack or debris. If nothing is found I'd just buy a new gasket kit and start over, about $6. Be 100% sure you sequence the gaskets exactly according to the instructions and tighten with a wrench. Keep in mind the threads are plastic so you could strip them if overtightened. The gasket and the fill valve hole are pretty much standard.