Since you talk top tube length I assume you mean forward extension.
If so, the longest standard extension is 140mm, and I prefer staying below 120, but fit trumps all. Don't worry about handling, that's more a question of weight distribution then stem length per se. In any case, you already have the frame so focus on getting the best possible fit using that. If a decent fit isn't possible, then you can shop for a more suitable frame.
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