Basically, you do not have to know the size of a string to append it another string.
There are a bunch of ways to do that:
string a = "a";
string b = "b";
StringBuilder sb = new StringBuilder(a);
string concat1 = a + b;
string concat2 = string.Concat(a, b);
string concat3 = string.Format("{0}{1}", a, b);
string concat4 = $"{a}{b}";
string concat5 = sb.Append(b).ToString();
All
concat
variables will hold the same value, which will be the concatenation of both string variables a and b.