I have been trying to change tmpfs size in the tmpfs service but I just changed the command in the service file, but is there a cleaner way to do this? And same with zramen init file where i need to change the size, but I'm not sure how to do it without just changing the service file itself that i assume will be overwritten by a possible update?
Probably not, the alternative would be (assuming you're on openrc) to create a conf.d/zramen for init.d/zramen to source, which (for the '-s <uint>' argument you want) is overkill.
I am thinking about ways to add easier service configuration to Dinit in a future version.
For the moment you can override the distro-provided service description, assuming it is located in /usr/lib/dinit.d, by copying it to /etc/dinit.d and making whatever changes you like there (this information, incidentally, is in the documentation). That way your changes shouldn't be overwritten by any update. The downside is that if there are fixes or other changes to the distro-provided service description (eg if they add a dependency or so on) you won't automatically get those fixes in your edited copy.
I've got a question, do you know where zramen's config file is? I've been looking but can't seem to find it.