Excerpt from the C99 standard, normative annex F (The C++-standard does not explicitly mention this annex, though it includes all affected functions without change per reference. Also, the types have to match for compatibility.):

IEC 60559 floating-point arithmetic

F.1 Introduction

1 This annex specifies C language support for the IEC 60559 floating-point standard. The IEC 60559 floating-point standard is specifically Binary floating-point arithmetic for microprocessor systems, second edition (IEC 60559:1989), previously designated IEC 559:1989 and as IEEE Standard for Binary Floating-Point Arithmetic (ANSI/IEEE 754−1985). IEEE Standard for Radix-Independent Floating-Point Arithmetic (ANSI/IEEE 854−1987) generalizes the binary standard to remove dependencies on radix and word length. IEC 60559 generally refers to the floating-point standard, as in IEC 60559 operation, IEC 60559 format, etc. An implementation that defines __STDC_IEC_559__ shall conform to the specifications in this annex.356) Where a binding between the C language and IEC 60559 is indicated, the IEC 60559-specified behavior is adopted by reference, unless stated otherwise. Since negative and positive infinity are representable in IEC 60559 formats, all real numbers lie within the range of representable values.

So, include <math.h> (or in C++ maybe <cmath>), and test for __STDC_IEC_559__.

If the macro is defined, not only are the types better specified (float being 32bits and double being 64bits among others), but also the behavior of builtin operators and standard-functions is more specified.
Lack of the macro does not give any guarantees.

For x86 and x86_64 (amd64), you can rely on the types float and double being IEC-60559-conformant, though functions using them and operations on them might not be.

How to guarantee exact size of double in C?

Use _Static_assert()

#include <limits.h>

int main(void) {
  _Static_assert(sizeof (double)*CHAR_BIT == 64, "Unexpected double size");
  return 0;
}

_Static_assert available since C11. Otherwise code could use a run-time assert.

#include <assert.h>
#include <limits.h>

int main(void) {
  assert(sizeof (double)*CHAR_BIT == 64);
  return 0;
}

Although this will insure the size of a double is 64, it does not insure IEEE 754 double-precision binary floating-point format adherence.

Code could use __STDC_IEC_559__

An implementation that defines __STDC_IEC_559__ shall conform to the specifications in this annex` C11 Annex F IEC 60559 floating-point arithmetic

Yet that may be too strict. Many implementations adhere to most of that standard, yet still do no set the macro.

would there be some standard way to guarantee size of a floating point type or double?

The best guaranteed is to write the FP value as its hex representation or as an exponential with sufficient decimal digits. See Printf width specifier to maintain precision of floating-point value