Simple Stack Implementation in C
It looks good. Here are a few things you can add to it or change:


You don't have to define true/false yourself. It's unfortunate that bool is not a built-in type in C, but the Standard C Library at least provides an #include file that gives  you those definitions in a portable way. So you can #include &lt;stdbool.h&gt; and it will define the type bool and true/false constants.
You can combine the structure declaration + typedef into the same declaration:

typedef struct stk 
{
    char content[MAXSTACKLENGTH];
    int top;
} stack;


Most C programs you'll find use this more compact notation of typedef struct.
You can simplify statements like this and others:


int isEmpty(stack *s)
{
    if (size(s) &lt;= -1)
    {
        return true;
    }
    else if (size(s) &gt; -1)
    {
        return false;
    }
}



By simply returning the condition:

int isEmpty(stack *s)
{
    return size(s) &lt;= -1;
}


The result of a comparison is always a boolean, so it can be returned directly if the function also returns true/false. This is another pattern that you will see used a lot. It makes code more concise.
Comparing against true/false in a conditional is not very usual:


if (isFull(s) == true)



You gave good descriptive names to your functions, so you can just test them like this:

if (isFull(s)) { ...


And to test if not full, the more usual way is to apply the ! (logical not) operator:

if (!isFull(s)) { ... // Reads "if not isFull"

size() currently return -1 for an empty stack. That's unusual. It doesn't make sense for a size to be negative. I'd expect an empty stack to have size 0. You should change that.
I suggest having a init_stack() function that sets the expected initial invariants when a new stack is declared. Better than having to remember to manually set top to -1, for example.
Since you already have a custom pause() function, you don't have to use system("pause"). The latter is non-portable and not recommended for use in real-life programs due to security risk it might incur. Best if you avoid it.
A minor nitpick, you're mixing different styles of curly brace positioning. Some are on the same line as the parent conditional, some are on their own line. This creates a break in your style, which is a little annoying. It would be nice if you kept them consistent, independent of which style you decide for.