Using Codesys 3.5.18.30
I am trying to work with an array of variable length in one dimension, but known in the other dimension.
For example, using an array that represents a list of items, each with an attribute profile.
I do not know how many items will be in the list, but I know that every item has 5 attributes, like size, weight, etc.

Codesys does not seem to be able to accept this.
I have referenced Data Type: ARRAY OF documentation page, which is not explicit if variable & fixed length array dimensions can be mixed.
In the VAR_IN_OUT scope, I initially tried
MY_ARRAY : ARRAY [*, *] OF REAL;
which was no problem, of course. I then modified it to
MY_ARRAY : ARRAY [*, 1..5] OF REAL;
which throws
C0006: ', or ]' expected instead of '..'
among consequent downstream errors.

I then used the Auto Declare -> Array Wizard, which accepted (asterisk)-(asterisk) and 1-5 for 2-dimensional lower and upper bounds.
It output:
MY_ARRAY: ARRAY[*..*, 1..6] OF INT;
which throws the same error stream.

What's the deal here? Is this possible?

Arrays with variable lengths are a bad idea.
This makes your system unstable and can lead to crashes.
Declare your array large enough to reserve enough memory

VAR CONSTANT
size : INT := 10;
END_VAR

My_Array : ARRAY[1..SIZE , 1..5] OF REAL;