I estimate it to be about 4.5 to 5 inches long at most. Seems very "camera-shy." I believe it has a lovely, mellifluous song with many high-pitched tones in a rather rapid succession. Had to use the 12x optical zoom on my Cyber-shot camera, thus the graininess. Photos taken this evening, 15 May, in my suburban North Texas backyard.
Hah! Now we'll have to wait for someone with more knowledge on these birds to help us out. By my experience what I've seen; those birds are about the Eastern Phoebes in size as well as behaviors. They're more colorful with the yellow color where Phoebes don't have.
It's one of the Empidonax flycatchers. No way of telling which, without recordings of the call and/or song. Listen to each of these online, and see which best matches the song you heard: