Firefox 4以上が対象。リンクが多いとハング歯科寝ないので注意

linkLarger.user.js

// ==UserScript==
// @name           linkLarger
// @namespace      http://gist.github.com/teramako/
// @description    リンクを大きくするお！！ 【注意】超重たい
// @include        http://*
// ==/UserScript==

const C = "linkLarger-link";
GM_addStyle(<><![CDATA[
.linkLarger-link {
  -moz-transition-property: font-size !important;
  -moz-transition-duration: 0.3s !important;
  font-size: 300% !important;
}
]]></>.toString());

function getAnchors() {
  var a = document.getElementsByTagName("a");
  for (let i=0, elm; elm = a[i]; i++){
    if (elm.classList.contains(C)){
      elm.classList.remove(C);
    }
    yield elm;
  }
}

function getTop3NearAnchor(x, y) {
  var ds = [];
  for (let elm in getAnchors()) {
    let {bottom: bottom, top: top, height: height, left: left, right: right, width: width} = elm.getBoundingClientRect();
    if (bottom < 0 || top < 0 || left < 0 || width < 0)
      continue;
    let elmX = left + width / 2,
        elmY = top + height / 2;
    let d = Math.sqrt(Math.pow(x - elmX, 2) + Math.pow(y - elmY, 2));
    ds.push([d, elm]);
  }
  ds.sort(function(a, b) a[0] - b[0]);
  return ds.slice(0, 3);
}

for (let elm in getAnchors()){
  elm.style.MozTransitionProperty = "font-size";
  elm.style.MozTransitionDuration = "0.5s";
  elm.style.fontSize = window.getComputedStyle(elm,"").fontSize;
}
document.addEventListener("mousemove", function(event) {
  getTop3NearAnchor(event.pageX, event.pageY).forEach(function($_){
    let elm = $_[1];
    elm.classList.add(C);
  });
}, false);