List item 2.2.2) in section 3.3 Non-Scalar Evaluation (aka 'Array expressions') in ODF 1.3, part 4, states:
<quote>
The result matrix is rectangular, sized with the maximum number of rows and columns from all non-scalar arguments.
</quote>
And the standard gives an example in Note 8:
<quote>
={1;2}+{3;4;5}	=> {4;6;#N/A}
</quote>

This formula in matrix mode (Ctrl+Shift+Enter) results in 1 row with the two elements 4 and 6 in LibreOffice.

To get the number of columns independent from the marked area and from spill behavior use the COLUMNS function:
Enter
=COLUMNS({1;2}+{3;4;5})
and finish with Ctrl+Shift+Enter.

LibreOffice results value 2, Excel results value 3.