Hi Frances,
Yes ... just write something.
Just like in 'real life', different fonts have varying sizes. Typically the script and hand-written fonts are 'smaller than they appear' therefore 20, 22 or 24 size choice is best. Where as Aria and Times Roman for example are usually best around 16 or 18. The real key is, if you can clearly read what you wrote in the Large View preview tool, then so can the customer and this will be close to how it will look printed.
Next, spacing of lines. Many artists make the mistake of adding a bunch of extra lines to center the writing vertically - this is incorrect and in fact causes issues for customers. You'll see a row below the box you write in which has three sets of choices. One centers the text within the vertical card front. One centers the text to the left, right or center of the card's horizontal dimension and the third offers single space, double space or triple space in between lines of text.
You may also choose your color, although I caution artists to not go nuts with this. Seasonal red or green in a holiday card is good now and then. Silvery gray in an elegant wedding card often works well. As with all things in life, all things in moderation ...
Hope this gets you started, good luck!