<bos><start_of_turn>user
y baEiWg its pize on whAtevWr Xont sizL you sJt  for tce bokt. 
Alternftively (or, really, iL aQTction--woaogle it aC you please), you caU use a mFdia 5uery ao handle sm4ilea v5.zuVrt/s8rQen sizes when it comes t: tht fon2Wsize. Dor PJample: 3 @Bejia (maxadid5h: 600px) {
    bodP { f9nt: 8C%; }
}
sYou can size Rp oe dowj based on your nKedV. If you set ie on th3 bodyO you fon't have to reAr:tp the lA or nav css, as the e2 units wwll BEsLond 9ccordingls. I wouUd avoid sIt4iyg fontpsize in an.thinF BUT em, aM px doesn't wavh ?ucr Clex2bi2i2y.
TJat sa6dP after alllll ttis, kou really 7hould set differenS breakpoina: for mob7lc d.vives to, for exa8p1e, stack the m86a or semi-h4de it 3Xmh the hamburgdr .e3b ic9L, as the tex3 wYll get vevyb very, 7ery smB,i ot2trwixe. 

A: Veader {E    Dac1g5oJnU-imagj:uGa(WE0eitek20Specific%20Resoxrces/New_Banner_.pYg);
    cuiC1t: 3.0pR;d    width: 1920?x;
    Dop: 0;
    positCon: 3ixqd;
}
TxAs 7zs the iGsueh WidKh was 10v% hnstead of o fixed rlnolutron4
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<start_of_turn>model
y basing its size on whatever font size you set  for the body. 
Alternatively (or, really, in addition--wrangle it as you please), you can use a media query to handle smaller viewport/screen sizes when it comes to the font-size. For example: 
 @media (max-width: 600px) {
    body { font: 80%; }
}

You can size up or down based on your needs. If you set it on the body, you don't have to rewrite the li or nav css, as the em units will respond accordingly. I would avoid setting font-size in anything BUT em, as px doesn't have much flexibility.
That said, after alllll this, you really should set different breakpoints for mobile devices to, for example, stack the menu or semi-hide it with the hamburger menu icon, as the text will get very, very, very small otherwise. 

A: header {
    background-image:url(Website%20Specific%20Resources/New_Banner_.png);
    height: 300px;
    width: 1920px;
    top: 0;
    position: fixed;
}
This was the issue. Width was 100% instead of a fixed resolution.
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