SizeInput: fix slider bug that broke functionality on mobile

We were, for some reason, preventing the default behavior of touch
events onTouchStart and onTouchEnd. This made the slider unusable with
drag actions on mobile. I can't remember why we did this to begin with,
probably something important...

The slider thumb size was also increased for better tap target size.

src/components/SizeInput/SizeInput.tsx

@@ -7,12 +7,10 @@ import "./SizeInput.css";
 type SizeInputProps = {};
 
 const handleFocus = (event: React.UIEvent<HTMLInputElement>) => {
-  event.preventDefault();
   event.currentTarget.focus();
 };
 
 const handleBlur = (event: React.UIEvent<HTMLInputElement>) => {
-  event.preventDefault();
   event.currentTarget.blur();
 };