Powershell: how to turn an array of objects into an array of integers?

The answer was provided in comments but to give it closure, you can expand the values of the objects using Select-Object -ExpandProperty. It's worth noting that your Sort-Object statement is not actually sorting by Length unless you specify the property to be sorted:

$length = dir -File | Sort-Object Length -Descending | Select-Object -ExpandProperty Length -First 5

Personal preference to expand the values, you can use ForEach-Object, the syntax is far easier in my opinion but there is no -First X here:

$length = dir -File | ForEach-Object Length | Sort-Object -Descending

$length type will be object[] (object array) even though it will contain an array of int64 values, this is default type for arrays in PowerShell, you can however provide a desired output type either by type casting or hard typing. Looking into the Cast Operator [..] is a good starting point.

# type casting
$length = [Int64[]] (dir -File | ....)

# hard typing
[Int64[]] $length = dir -File | ....

For the latter, $length is constrained to be assigned int64 values or values that can be type coerced to int64, i.e.:

[int64[]] $length = 0..10

# can be assigned an `int32` and the array element will be converted to `int64`
$length = 1
$length[0].GetType() # => `int64`

# can be assigned a `string` which contains for example a decimal value
$length = '1.1', 2
# gets converted to int64 (no longer a decimal value)
$length[0] # => 1

# cannot be assigned a `string` which does not contain a type of numeric value
$length = 1, 'hello' # => ...Cannot convert value "hello" to type "System.Int64"...

While the former only enforces the type of the array elements however a new array of different type can be assigned:

$length    = [int64[]] (0..10)
$length[0] = 'hello' # => ...Cannot convert value "hello" to type "System.Int64"...

$length = 'new', 'object', 'array' # succeeds

Unfortunately, you can't use Memeber Access Enumeration for Length because it's also a property of the collection (Unlike, for example, (gci -af).LastwriteTime). But another way to obtain your desired array is:

$Length = (gci -af).Foreach({$_.Length}) | sort -Descending

Without casting, $Length is the generic object array:

PS > $Length.GetType()

IsPublic IsSerial Name                                     BaseType
-------- -------- ----                                     --------
True     True     Object[]

But all its elements are the same type:

PS > ($Length | gm).TypeName | select -Unique
System.Int64