When you have been doing postcard always, you might have known already that the size you have chosen really matters. It might even make or break the overall impact of the postcard that you are creating. Given that, it has always been safe that when deciding for your postcard size, you always stick to a standard postcard size.
The standard postcard size is what you usually see when receiving one. It is neither too small nor too big. It is just enough to be seen by a normal reader and has enough space to contain the message you want to get across clearly.
This size is perfect if the event is just the usual event. It might be important, but not that important to be written in an upsized postcard. It is also perfect if you have some few reminders in it like the date, the name of the event, the name of the recipient, and some little details. If you also want to place some images or graphics which are relevant, but not too complicated, the standard size is already enough.
Now, if you want to really emphasize that the person needs to attend the event, you can go beyond the standard size. A very huge size will make that person realize that it is indeed a very important event. If you also have lots of things to say, a bigger one is a much better option. In short, the size you are to decide upon depends on the call of the situation.
Finally, you have to consider the budget for printing. If you have enough funds only, the standard size is the perfect choice. It can contain all essential details without actually asking much from you. If you go for bigger sizes and you need to print a lot, be prepared to spend some more.
If you want to know some other details about postcards, including business postcard templates, check out postcardsize.net