Custom size of embed Videos

Hello. I was trying to insert custom size of embed videos on my website and everything was good in the text editor, but when I updated my Post, the video didn't stay custom size (it remained in the original size) and I think that is probably a theme problem. I wouldn't like to use an plugin to do this so I came to this forum to look for a help. Someone already passed trough this and solve this problem or could help me to find the problem? Best reguards, Você Deveria Saber (my website).