I know that (a handful of) every thing is an amount of it, but I have s.th to ask about it.
1-How much is a handful of s.th? Is it a small amount of that thing or a big sum?
2-What kind of verb I shall use with A HANDFUL OF …? A singular verb or a plural one?
A handful can mean different things depending on the context. It generally means a small amount of something.
So for example, you might hear something such as "Only a handful of participants were chosen from the thousands that entered". Obviously a group of people won't fit into ones hands, so in that context it is used to refer to a small number of people.
If something can physically fit into a hand, for example jelly beans, then a handful is likely to literally be a similar amount to which can fit into a hand.
Also, use plural verbs. Singular ones don't make sense with its usage.