But I always can change that later on when I am some further with my library
Now, when there is a small amount of code, or later when there is more. I know when I'd start using better names.
int declength = sizeof(arrayc);
The arrayc variable (dumb name) is an int. All ints are 2 bytes. That's what sizeof() is telling you.
So, you are reserving space for one character and the trailing NULL.
since I might be able to use sizeof(); to determine the length of the variable
No, you can't.
An int can hold a value between -32768 and +32767. If you can't figure out the size of an array necessary to hold the string representation of an int from that information, we can't really help you anymore.