On https://www.zabbix.com/documentation/2.2/manual/config/items/item

1) we don't have size description for Numeric (float), but we have it for Numeric (unsigned).
According to zabbix db schema (MySQL) for float we use "double(16,4)" - http://dev.mysql.com/doc/refman/5.5/en/floating-point-types.html
But need to re-ask C devs to be sure.

2) it worth to also explicitly mention with a short sentence that: "Numeric (float) can store negative values."