metamask-extension/ui/components/component-library/base-avatar/base-avatar.scss

.base-avatar {
  --avatar-size: var(--size, 16px);

  &--size-xs {
    --size: 16px;
  }

  &--size-sm {
    --size: 24px;
  }

  &--size-md {
    --size: 32px;
  }

  &--size-lg {
    --size: 40px;
  }

  &--size-xl {
    --size: 48px;
  }

  height: var(--avatar-size);
  width: var(--avatar-size);
  border-radius: 100%;
  overflow: hidden;
  display: flex;
  align-items: center;
  justify-content: center;
}