Answers for "how to increase scroll bar size in css with overflow auto and height"

scrollbar thumb height css

::-webkit-scrollbar-button {
  height: 12px;
}
::-webkit-scrollbar {
    width: 12px;
}
/* Track */
::-webkit-scrollbar-track {
    -webkit-box-shadow: inset 0 0 6px rgba(0,0,0,0.3);
    -webkit-border-radius: 10px;
    border-radius: 10px;
}
/* Handle */
::-webkit-scrollbar-thumb {
    -webkit-border-radius: 10px;
    border-radius: 10px;
    background: rgba(255,0,0,0.8);
    -webkit-box-shadow: inset 0 0 6px rgba(0,0,0,0.5);
}
::-webkit-scrollbar-thumb:window-inactive {
background: rgba(255,0,0,0.4);
}

overflow scrollbar with variable height

/* The element which contains one/many content sections */
.container {
  display: flex;
  flex-direction: column;
}

/* The container's content you want to add an overflow to */
.container .scrollable-content {
  flex-grow: 1;  
  overflow: auto;
}