In my rules, do I refer to this tag as "ns2:Height" or as "Height" somehow accounting for the namespace?

You can assign namespace prefixes using the namespaces option, for example namespaces => { 'http://www.example.com/foo' => 'ns2' }, and then use the ns2: prefix in your rules. Or, you can use the same option to map different namespaces to the same prefix, if that is acceptable in your application (!), in the way I showed below (in that case the empty prefix).

Can I have rules that just deal with Height, Length, Width, Weight, etc?

Yes, you can specify multiple tag names for a rule with |. You can also use regexes to specify rules, as in '/foo/' => ... (although I don't think the two can be combined). Here's an example:

use warnings;
use strict;
use XML::Rules;
use Data::Dump;

my $parser = XML::Rules->new(
    stripspaces => 3|4, warnoverwrite => 1,
    namespaces => {
        'http://www.example.com/foo' => '',
        'http://www.example.com/bar' => '',
        '*' => 'die' }, # don't allow other namespaces
    rules => [
        'Height|Length|Width|Weight' =>
            sub { lc $_[0] => $_[1]->{_content} },
        'root' => 'pass',
        _default => sub { die "Unknown tag $_[0]" },
    ] );

dd $parser->parse(<<'END_XML');
<root xmlns="http://www.example.com/bar"
    xmlns:foo="http://www.example.com/foo" >
    <foo:Height Units="inches">1.00</foo:Height>
    <foo:Length Units="inches">1.00</foo:Length>
    <foo:Width Units="inches">1.00</foo:Width>
    <foo:Weight Units="pounds">0.01</foo:Weight>
</root>
END_XML

__END__

{ height => "1.00", length => "1.00",
    weight => 0.01, width => "1.00" }
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In this code, I've made the _default rule be to die, that way I can make sure during development that I haven't overlooked a tag in the input XML. But you can of course also specify any _default rule you like.

By the way, since these questions are going a bit off topic from the original post, it might be best to start a new thread on your questions about XML::Rules, showing your code, enough example input to reproduce, etc. (Short, Self-Contained, Correct Example).