bigger sidebar icons?

Something simple like just .tab-stack{ transform: scale(1.2) } might work mostly fine. But I think it might lead to some small overflow problems. Alternatively you could try this:

#tabbrowser-tabs[orient="vertical"]{
  --uc-tab-icon-size: 24px; /* modify this to change tab icon size */
  --tab-min-height: max(calc(24px + (var(--uc-tab-icon-size) - 16px)),calc(1.7 * 1em));
}
#tabbrowser-tabs[orient="vertical"]:not([expanded]) .tab-background{
  margin-inline: auto !important;
  width: calc(var(--tab-min-height) + 2px) !important;
  aspect-ratio: 1;
}
.tab-icon-image{
  width: var(--uc-tab-icon-size) !important;
  height: var(--uc-tab-icon-size) !important;
}
#tabbrowser-tabs[orient="vertical"]:not([expanded]) .tab-content{
  padding-inline: 0 !important;
  margin-inline: auto;
}
#tabbrowser-tabs[orient="vertical"]:not([expanded]) .tab-close-button{
  inset-inline-start: calc(16px - var(--uc-tab-icon-size)) !important;
}

The tab scrollbar you can probably make bigger simply like this:

#tabbrowser-arrowscrollbox[orient="vertical"]{
  --uc-scrollbar-width: auto;
}
scrollbox[orient="vertical"]{
  scrollbar-width: var(--uc-scrollbar-width) !important;
}