Get attribute of a field from a VHDL record type

You cannot use the 'range attribute on a type ^†, which is what you are trying to do in your code. If you were to do something like this:

signal big_record_instance : big_record_t;
signal ex : unsigned(big_record_instance.field_a'range);

It should work, because you are now trying to get the range of an instance, not a type.

An alternative if you don't have an instance might be to have your width based on constants in the same package that your record type is defined, something like this:

constant field_a_width : integer := 16;

type big_record_t is record
        field_a : std_logic_vector(field_a_width-1 downto 0);
        field_b : std_logic_vector(23 downto 0);
end record;

signal ex : std_logic_vector(field_a_width-1 downto 0);

Or perhaps

constant field_a_width : integer := 16;

subtype field_a_type is std_logic_vector(field_a_width-1 downto 0);

type big_record_t is record
        field_a : field_a_type;
        field_b : std_logic_vector(23 downto 0);
end record;

signal ex : field_a_type;

† see exception in comments