Ah. All i see is a title. Welp, lemme see what I can do.

There are 2 fixes that I can think of to fix this problem.

1.) Play Minecraft in fullscreen.

2.) Install OptiFine.
It is really simple to install, and will work on all vanilla servers. Yes, it is a mod, but does not need a launcher. This mod also allows to help with lag issues, but amazingly allows you to fit your window size in game! here is a link to optifine's website:

http://optifine.net/downloads

I hope this helps with your problem.
-Agentdid127