The question "how big is a" is incomplete, but based on the search results, it looks like common inquiries might include the size of paper sizes like A3 or A4, or units like an acre or a micrometer (µ). Here are some reference points:

• A3 paper size is 297 x 420 mm (about 11.7 x 16.5 inches).
• A4 paper size is 210 x 297 mm (about 8.3 x 11.7 inches).
• An acre is a unit of land area equal to 43,560 square feet, roughly 208.7 feet by 208.7 feet square, or about 75% of an American football field.
• The "µ" symbol represents a micrometer (one millionth of a meter), which is extremely tiny, smaller than even the diameter of a cobweb strand.

If there is a specific object or unit that the query is about, please specify so I can provide exact details. For now, if the question was about paper size or acreage size, the above details provide a clear sense of scale. Would you like information on a specific "how big is a..." topic? For example, paper sizes, land measurements, or something else?