Is it possible to create a custom sized variable type in c?

You can create custom types of all sorts, and if you want a "integer" type that is 16 bytes wide you could create a custom struct and pair two long longs together. But then you'd have to implement all the arithmetic on those types manually. This was quite common in the past when 16 bit (and even 32 bit) machines were most common, you'd have "bigint" libraries to do like 64-bit integer math. That's less useful now that most machines are either 64 bit or have long long support natively on 32 bit targets.

You used to see libraries with stuff like this quite often:

typedef struct _BigInt {
    unsigned long long high;
    unsigned long long low;
} BigInt;

// Arithmetic functions:

BigInt BigIntAdd(BigInt a, BigInt b);

// etc.

These have faded away somewhat because the current typical CPU register width is 64 bits, which allows for an enormous range of values, and unless you're working with very specialized data, it's not longer "common" in normal programming tasks to need values outside that range. As @datenwolf is explicit and correct about in the comments below, if you find the need for such functionality in production code, seek out a reliable and debugged library for it. (Writing your own could be a fun exercise, though this sort of thing is likely to be a bug farm if you try to just whip it up as a quick step along the way to other work.) As Eric P indicates in the comments above, clang offers a native way of doing this without a third party library.

(The weird ambiguities or equivalencies about the widths of long and long long are mostly historical, and if you didn't evolve with the platforms it's confusing and kind of unnecessary. See the comment on the question about this-- the C standard defines minimum sizes for the integer types but doesn't say they have to be different from each other; historically the types char, short, int, long and long long were often useful ways of distinguishing e.g. 8, 16, 32, and 64 bit sizes but it's a bit of a mess now and if you want a particular size modern platforms provide a uint32_t to guarantee size rather than using the "classic" C types.)

Obviously you can. By preference you should not use string, because computations with those will be a lot more complicated and slower.

Also, you may not want to use bytes, but the 2nd largest datatype available on your compiler, because detecting overflow can be cumbersome if you're using the largest datatype.