hm, I think I’ve found the right bit in the css file. Copied it to my custom.css, but changes are having no effect.
The { width: 170px; } tags are my additions.
/* base : forms
/* ------------------------------------ */
input, textarea, button, select, label { font-family: inherit; }
.themeform input, { width: 170px; }
.themeform textarea,
.themeform button,
.themeform select,
.themeform label { font-size: 14px; }
.themeform input::-moz-focus-inner,
.themeform button::-moz-focus-inner { border: 0; padding: 0; }
.themeform input[type="text"], { width: 170px; }
.themeform input[type="password"],
.themeform input[type="email"],
.themeform input[type="submit"], { width: 170px; }
.themeform button,
.themeform textarea { margin: 0; -moz-box-sizing: border-box; -webkit-box-sizing: border-box; box-sizing: border-box; -webkit-border-radius: 0; border-radius: 0; }
.themeform input,
.themeform textarea,
.themeform button { -moz-appearance: none; -webkit-appearance: none; -moz-transition: all .2s ease; -webkit-transition: all .2s ease; transition: all .2s ease; }
.themeform input[type="checkbox"] { -moz-appearance: checkbox; -webkit-appearance: checkbox; }
.themeform input[type="radio"] { -moz-appearance: radio; -webkit-appearance: radio; }
.themeform label { font-weight: 600; color: #444; }
.themeform input[type="text"], { width: 170px; }
.themeform input[type="password"],
.themeform input[type="email"], { width: 170px; }
.themeform textarea { background: #fff; border: 2px solid #ddd; color: #777; display: block; max-width: 100%; outline: none; padding: 7px 8px; }
.themeform input[type="text"]:focus,
.themeform input[type="password"]:focus,
.themeform input[type="email"]:focus,
.themeform textarea:focus { border-color: #ccc; color: #444; -webkit-box-shadow: 0 0 3px rgba(0,0,0,0.1); box-shadow: 0 0 3px rgba(0,0,0,0.1); }
.themeform label .required { color: #3b8dbd; }
.themeform input[type="submit"], { width: 170px }
.themeform button[type="submit"] { width: 170px; background: #3b8dbd; color: #fff; padding: 8px 14px; font-weight: 600; display: inline-block; border: none; cursor: pointer; -webkit-border-radius: 3px; border-radius: 3px; }
.themeform input[type="submit"]:hover,
.themeform button[type="submit"]:hover { background: #444; }