How to make portfolio popup bigger?

It can't be done via theme options, but you can use this custom css (just add it to the bottom of style.css via menu Appearance -> Editor):

div#portfolio_box {
    max-width: 100%;
}
.mfp-content div.container {
    max-width: 90%;
}
#portfolio_box img {
    display: block;
    margin: 0 auto;
}

or this one

div#portfolio_box {
    max-width: 900px;
}
.mfp-content div.container {
    max-width: 900px;
}
#portfolio_box img {
    display: block;
    margin: 0 auto;
}