How to increase the size of the multiple choice field (Check Box)

Yes, this can be achieved through custom CSS. For example, you could add the following custom CSS into your form to modify the appearance of the check boxes:

https://www.jotform.com/help/117-How-to-Inject-Custom-CSS-Codes

input.form-checkbox { display: none; }
.form-checkbox-item label {
    line-height: 18px;
    position: relative;
    margin-left: 0;
    float: left;
    text-indent: 27px;
}
.form-checkbox-item label:before {
    content: '';
    position: absolute;
    display: inline-block;
    vertical-align: baseline;
    margin-right: 4px;
    -moz-box-sizing: border-box;
    -webkit-box-sizing: border-box;
    box-sizing: border-box;
    left: 4px;
    width: 20px;
    height: 20px;
    border: 1px solid #158;
    border-radius: 2px;
}
.form-checkbox-item label:after {
    content: '';
    position: absolute;
    z-index: 10;
    display: inline-block;
    opacity: 0;
    top: 4px;
    left: 8px;
    width: 12px;
    height: 12px;
    background: #158;
}
.form-checkbox-item input:checked + label:after {
    opacity: 1;
}

You can also play with the sizes indicated in red to either increase or decrease the checkboxes. Please try it out, and let us know whether this works to your satisfaction.