JSFiddle - React, Tailwind, and code Playground

HTML

<div class="size-input-wrapper">
    <label for="inputValidation">Enter size:</label>
    <input type="number" id="inputValidation" placeholder="size"/>
    <span class="pxSpan">px</span>
 </div>

CSS

.size-input-wrapper {
    max-width: 208px;
    margin: auto;
    position: relative;
    display: inline-block;
}
#inputValidation {
    padding-right: 35px;
}
.pxSpan {
    position: absolute;
    top: 19px;
    right: 10px;
}