Does Unary + operator do type conversions?

Unary + performs integer promotions on its operand, we can see this by going to the draft C99 standard section 6.5.3.3 Unary arithmetic operators which says (emphasis mine going forward):

The result of the unary + operator is the value of its (promoted) operand. The integer promotions are performed on the operand, and the result has the promoted type.

and section 6.3.1 Arithmetic operands says:

If an int can represent all values of the original type, the value is converted to an int; otherwise, it is converted to an unsigned int. These are called the integer promotions.⁴⁸⁾ All other types are unchanged by the integer promotions.

Note that all other types are unchanged by the integer promotions and thereofore double stays a double. This would also hold for float as well, which would not be promoted to double.

Also note that using %d for the result of sizeof is undefined behavior since the result is size_t. the proper format specifier would be %zu.

When smaller types are involved in an expression with larger types (for example, char is smaller than short which mostly is smaller than int which may be smaller than long), the involved types are promoted to the larger tyoes.

So yes, when you use the unary + operator, you get an int, because int is the natural integer type in C.

Regarding the double type, the natural floating point type for C is double, which is why there's no promotion on values or variables that already are of type double.

The sizeof operator have nothing to do with this.

Operator unary + triggers "initial" usual arithmeric conversions, so all integer operands, which type has lower rank than rank of int and unsigned int are promoted to int (or unsigned int if int type does not cover all values of type being promoted on that implementation).