Since vertexShader is a pointer to a char, *vertexShader is of type char, and thus sizeof(*vertexShader) is equivalent to sizeof(char). In order to use sizeof to get the length of the string, you have to make vertexShader an array, and take the size of the array. I also recommend you insert new-line characters at each new line so that you can get some sensible line numbers from the error messages.
const char vertexShader[] = ""
"void main(void){\n"
" gl_TexCoord[0] = gl_MultiTexCoord0;\n"
" gl_Position = ftransform();\n"
"}\n";
int vertexSize = sizeof(vertexShader);