A video watermark is a compact branding image, so a complete horizontal wordmark often contains too much detail. Use a simple symbol, a clear initial, or the compact version of your logo. The important question is whether its shape remains recognisable when it is displayed over video, not whether every part of the company name fits inside a tiny square.
YouTube's branding help asks for a square watermark with a minimum of 150×150 pixels and a file smaller than 1 MB. This page starts at that minimum with PNG output. Check the downloaded file size because PNG does not reduce through a JPEG quality slider, and a complex image can still be unexpectedly large.
Start with a source that already has the intended transparent background. The tool can preserve existing transparency when exporting PNG, but it does not trace a logo, remove white pixels, or separate a symbol from its background. Obtain a transparent master from your designer if the background is wrong.