Installed Debian 9.6 in VirtualBox 25GB vhd and partitioning said that:
"Maximum size for this partition is 26.8GB"
"New partion size 26.8GB".
df shows 25669860, which seems to be correct.
Where did that extra 1.6GB come during installation?
That's because the installer displays the size in decimal gigabytes, whereas other utilities use binary gigabytes.
In bytes, 25 * 2^30 = 26,843,545,600; or, in larger units, 25 GiB = 26.8 GB.
One binary gigabyte (sometimes called a gibibyte) is 2^30 = 1,073,741,824 bytes.
One decimal gigabyte is 10^9 = 1,000,000,000 bytes.
(This is an old dilemma. Generally, the prefixes kilo-, mega-, giga-, tera- etc. refer to powers of 10. By abuse of language, it became usual to use them loosely to refer to powers of 2 which have values close to the corresponding powers of 10. Commonly, the binary meaning is almost always meant when speaking of memory capacity; and the decimal meaning is almost always meant when speaking of bandwidth. But for disk storage capacity some utilities default to the decimal meaning, and others to the binary meaning.)