Consistent return type for size()

## Description
As mentioned in #2641, the `math::size()` overloads should have a return type consistent with Eigen's `size()` member to avoid compiler errors when calling code such as:

```
  Eigen::VectorXd a(2);
  a << 1, 2;

  std::max(a.size(), stan::math::size(a));
```

Which resolves to:
```
std::max(size_t, Eigen::Index);
```

#### Current Version:
v4.4.0
