[kirab]

Well..

  C and C++ offer a wide variety of arithmetic types.
  Double precision is not required by the standards
  (except by the optional annex F of C99, covering
  IEEE 754 arithmetic), but on most systems, the
  double type corresponds to double precision.
  However, on 32-bit x86 with extended precision
  by default, some compilers may not conform to
  the C standard and/or the arithmetic may suffer
  from double-rounding issues.

[daemin]

So a reasonable answer could say - usually 8 bytes - but sometimes with extended precision it could be 10 bytes, etc...

I am actually kind of sick of the people that say these details are not important to being a good programmer. They absolutely are, knowing fundamentals of the machine, language, and environment that you are developing in is essential.

[scatters]

ap22213 (OP for this thread) was more precise than I was; they actually wrote "a double precision floating point variable" rather than writing "double", which could either be a shorthand for "double precision floating point" or be referring to a platform/language type (where that exists by that name).