Specify a location - town, city or countryside.
If country be more specific - are there restrictions. (A1,A2, or A3 zoning).
Are there neighbouring plots and what sizes are they?
Is there an existing dwelling on the site? Is the site serviced? A site with a septic tank needs a certain amount of space for the percolation area etc.
Generally speaking it is 1/3 to 1/2 an acre.
1/2 acre is min. comfortable size for country IMO.