My old monitor with a bit of a flicker finally conked out. I had another monitor ready to go. On the new monitor the screen is not full. There's about 1" black bars on the sides and about 3/4" black bars it top and bottom. I cant remember what to do to get a full screen.
EDIT: I have win-7.Both monitors are HP-25". Set to 1920x1080(max).

Change the resolution of your desktop to fit the monitor.
https://www.wikihow.com/Change-the-Screen-Resolution-on-a-PC

EDIT -- if 1920x1080 is not listed as available, you may have to download monitor drivers from the HP website.

Did all that. Used recommended resolution.(1920x1080) Did not fill screen.

It sounds like your monitor may have a native resolution greater than 1920x1080 resolution then.

Do you have the model number? You may be able to find a pdf manual on the HP website.

You have a choice -- you can check the manual to see if there is an option in the monitor to stretch whatever resolution you're using to full screen, or you can make sure your monitor's native resolution is only 1920x1080 and that it's not a 4k monitor or some other resolution greater than 1920x1080.

Weird. Switched to smaller numbers ( 1680x1050 ) and it is filling the monitor.
Problem seems to be fixed but I don't get it.

That is strange.
Usually it's the other way around.
Did you check the refresh rate (Hz).
Maybe the monitor is trying to use 120 Hz or 144 Hz instead of 60 Hz.
If that's the case, the video card might not be able to sustain 1920x1080 at full screen and 144 Hz -- so switches to windowed. But it can do 1680x1050 and 144 Hz.

This is an interesting conversation... that is strange. Does the picture look distorted at all when you changed to 1680x1050? And I didn't know that about the refresh rate...

Does not look distorted. Refresh rate is 60.

I guess whatever works! Hope this one lasts a good long time for you! I have an old 19" HP monitor as a backup and for when I'm working on other computers. Wouldn't be without it.

What model is it?
If it's the same model as the older monitor, it should operate the same way.
You should be able to get 1920x1080 full screen.
What Hz is showing up when you have the monitor set to 1920x1080 and the picture is showing up windowed?

Old is HP-2509b
New is HP-2511x
Hz is 60.
Seems to be OK with lower numbers.

According to the ***user guide for the 2511x at HP*** it can only handle 60 Hz.
But it should handle 1920x1080 as long as it's set to 60 Hz.

If it's not displaying 1920x1080, you can check to make sure the computer is correctly detecting the monitor as the 2511x and not the 2509b.

Other than that I don't know why it would display 1920x1080 resolution as windowed.

If there is an option in the monitor's menu to set back to factory defaults, you could try that.

Thanks for info.