What is the memory difference between serializing a value as string and serializing a value as other data type in protobuf

Question

Let's say I have a variable a, which I want to assign a decimal. If my proto file is

syntax = "proto3";

message Test{
 string a = 1;
}

How much memory will that take and how much will be the difference if I change string a =1 to float a =1. Is there a documentation where you can see how much memory is assigned to different datatype?