Make offcanvas appear from bottom of page, with custom width. (set offcanvas to slide from left or right, so you have access to the width control)

style.css

/* replace #-off-canvas-3-450 with your offcanvas ID */

#-off-canvas-3-450 .offcanvas-inner {
    left: 50%;
}

#-off-canvas-3-450 .offcanvas-inner.oxy-inner-content {
    transform: translate(-50%,100%);
    -webkit-transform: translate(-50%,100%);
    display: block; /* depending on form, to prevent flex overflow issues */
}


#-off-canvas-3-450.oxy-off-canvas-toggled .offcanvas-inner {
    transform: translateX(-50%);
    -webkit-transform: translateX(-50%);
}

body.oxygen-builder-body #-off-canvas-3-450 .offcanvas-inner.oxy-inner-content {
    left: 50%;
    transform: translateX(-50%);
    -webkit-transform: translateX(-50%);
}