The problem is that currently there are many routes that currently have no signs. Any sign is better than none... Sure it's easier to spot larger signs, but are there really many people who are willing to travel portages covered with blowdowns and sometimes non-existent portage trails that aren't willing to use a map?
That being said.... Even if multiple signs are posted, I expect that they will still quickly tear and crumble because they're so weak. A larger durable sign might be useful. I know the Township of the Algonquin Highlands uses full sized metal signs for their campsites.
By the way, all the signs are white on the low maintenance portages because of age

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Jeff
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Algonquin Backcountry Recreationalists - Caring for Algonquin's Backcountry - abrweb.ca
My Algonquin Park Map - Accurate, Detailed, Free