Something tells me Dagur would choose a short one-
Dagur: Hey! I wouldn't... Okay... Okay, yeah, I would. Then I would drop something and ask him to pick it up. Mm, yeah, that'd be great. Hiccup in a skirt.
Viggo: Dagur, please pull yourself together. We have an audience. Anyhow, I would probably pick something long and elegant for Hiccup. He'd look lovely.
(Spoilers for RTTE) It frustrated me how there was never a canon explanation for why Dagur sent Heather adrift as kids. I don't think it was done with malicious intent. My reason for this is how much Dagur holds grudges. If he had hated her, then he probably would have continued to hate her up until turning good; however, as soon as he found out who she was, he didn't care that the reason she was even there to begin with was literally to kill him. He just wanted her back in his life. Something I also thought was interesting was how he was shown to be able to love others even before becoming a better person. As a villain, he could've viewed Heather as a pawn or a lackey, but he was extremely protective of her and brought her in on his actual plan for the Grimborns. So, no, I don't think he disliked her as a child. That leads me to thinking it's one of two reasons for why he did what he did.
The first is that it was an accident. I don't think Dagur's much older than Heather, so it could have been an instance of two children being left unsupervised and it not going well. Maybe he thought they would have fun on a little boat and then a big wave separated them. Maybe he assumed that she would enjoy getting some time alone to sail. An important part of this too in my opinion is how Dagur doesn't process things like most Vikings, so it could have made sense to him at the time and actually been very dangerous.
The second option is that he did want to send her away, but it was to protect her. This could have been from raiding Vikings or even their own Berserkers like Ansson. If Dagur knew he wasn't a strong enough fighter at the time to beat the older redhead, maybe he thought sending Heather away would keep her from getting hurt as well. Another option is if there was an illness on Berserker Island. Then, sending her odd could have prevented her from contracting it.
I've played around with a lot of these ideas in different stories, but I can never really settle on one. What do you all think?