My final cut pro videos aren't filling up the player on Youtube

ok so I was able to figure out why it wouldn't let me insert the image and it was cause the file size was to big so I had to just take a screen shot of the inspector and the viewer itself and not the whole screen, I'm use to windows cause they would tell you file size is to big instead I was left to try to figure it out. I really hope I'm not irritating you cause I greatly appreciate your help. I don't know how to find out the dimensions of my clips and I don't know how to make them match up in final cut pro

Thanks for replying, so after reading this I went through and looked at the inspector and found the spatial conform and it was fit and I changed it to fill which seemed to kinda fix it I think, but now it took out some of the top of the screen out so the at the end of the video you can't see the top of the waterfall in the video which is frustrating. so I took 3 screen shots and they are sitting on my desktop but when I try to insert them into my reply it won't let me insert the image, I click on the button and nothing happens no warning or telling me why I can't insert the image. I took the screen shots by holding command shift 3 I don't know if that has anything to do with why it won't work, I'm getting super frustrated cause it seems like I can't even do what I need to get the help I'm seeking and I don't know why cause I'm not use to apple products.

ok I found everything you just instructed me to change and although I couldn't make the clips exactly match I just changed it to closet thing I could and that was 1920x1080 and the clips are 1920x1440 so I hope that will be close enough. I greatly appreciate your help if you had not helped me I would have been left trying to figure it out and had no clue where to actually change the settings to get the desired effect. I made sure go through and mark all the comments helpful that I could and I tried to see if I could do anything else and I couldn't see anything. Thank you sooooooo much you are awesome Luis

sorry for the late response, I was away for the weekend and didn't have access to internet. my videos came from a GoPro Hero3. I didn't mean to have as 1920x1440 I think thats what final cut pro suggested when I imported them into final cut pro and I'm very new to final cut pro and video editing in general so I'm kinda learning things the hard way but I'm definitely open to any tips/tricks or tutorials cause when I start to edit I'm definitely winging it a lot of the time and watching youtube videos to get better.

oh ok I will definitely do the custom frame size so I can make them identical which would ideal.

here is a link to the video just in case people want to see what I'm talking about https://www.youtube.com/watch?v=i1ckdXTgk_o