CCNA, CCNP, MCSA, CCNA Final Exam, All Answer Test Module With 100/100

In C, I'm working on a "class" that manages a byte buffer, allowing arbitrary data to be appended to the end. I'm now looking into automatic resizing as the underlying array fills up using calls to realloc . This should make sense to anyone who's ever used Java or C# StringBuilder . I understand how to go about the resizing. But does anyone have any suggestions, with rationale provided, on how much to grow the buffer with each resize?

i want to insert a % character before after every letter in a string , but using StringBuilder to make it fast. for example, if a string is 'AA' then it would be '%A%A%' if it is 'XYZ' then it would be '%X%Y%Z%'