Hmm. Well, the whole code is a fixed width, and the images are set to take up certain percentages of that width. So in order to make the side boxes bigger, you're going to have to make the main box smaller, OR make the whole thing bigger.

edit: okay I'm not sure why the previews are being wonky here, but try the codes anyway

option a)



<div style="box-sizing: border-box; display: grid; grid-template-areas: 'big1 big1 small1a' 'big1 big1 small1b' 'big1 big1 small1c' 'textbox textbox textbox'; grid-gap: 5px; width: 285px; height: 325px; margin: auto; background: #222; padding: 5px;"> <div style="background-color: #222; background-image: url(https://78.media.tumblr.com/3494b438858b23b1f5c07c61c790b95e/tumblr_mzlgoduHS71qdoecso3_250.gif); background-size: cover; background-position: center; grid-area: big1;"> </div> <div style="background: url(http://www.insanejournal.com/userpic/33423928/4032417); background-color: #EEE; background-size: cover; background-position: center; grid-area: small1a;"> </div> <div style="background: url(http://www.insanejournal.com/userpic/33423930/4032417); background-color: #DDD; background-size: cover; background-position: center; grid-area: small1b;"> </div> <div style="background: url(http://www.insanejournal.com/userpic/33423932/4032417); background-color: #CCC; background-size: cover; background-position: center; grid-area: small1c;"> </div> <div style="background-color: #FFF; display: flex; flex-direction: column; justify-content: center; align-items: center; grid-area: textbox; font-family: candara, sans-serif; font-size: 12pt; color: black;"> <span style="font-weight: bold; letter-spacing: .5em; padding-left: 1em;">ROSE HATHAWAY</span> <span style="color: gray;">VAMPIRE ACADEMY</span> <span style="color: gray;"><a href="LINK">bio</a> | <a href=LINK>tags</a></span> </div></div>

option b)



<div style="box-sizing: border-box; display: grid; grid-template-areas: 'big1 big1 big1 small1a' 'big1 big1 big1 small1b' 'big1 big1 big1 small1c' 'textbox textbox textbox textbox'; grid-gap: 5px; width: 385px; height: 425px; margin: auto; background: #222; padding: 5px;"> <div style="background-color: #222; background-image: url(https://78.media.tumblr.com/3494b438858b23b1f5c07c61c790b95e/tumblr_mzlgoduHS71qdoecso3_250.gif); background-size: cover; background-position: center; grid-area: big1;"> </div> <div style="background: url(http://www.insanejournal.com/userpic/33423928/4032417); background-color: #EEE; background-size: cover; background-position: center; grid-area: small1a;"> </div> <div style="background: url(http://www.insanejournal.com/userpic/33423930/4032417); background-color: #DDD; background-size: cover; background-position: center; grid-area: small1b;"> </div> <div style="background: url(http://www.insanejournal.com/userpic/33423932/4032417); background-color: #CCC; background-size: cover; background-position: center; grid-area: small1c;"> </div> <div style="background-color: #FFF; display: flex; flex-direction: column; justify-content: center; align-items: center; grid-area: textbox; font-family: candara, sans-serif; font-size: 12pt; color: black;"> <span style="font-weight: bold; letter-spacing: .5em; padding-left: 1em;">ROSE HATHAWAY</span> <span style="color: gray;">VAMPIRE ACADEMY</span> <span style="color: gray;"><a href="LINK">bio</a> | <a href=LINK>tags</a></span> </div></div>