There are a lot of things to consider when printing letters and words. This blog looks at just one part. That of making the letters the correct size in relation to the others. This is a matter of developing awareness of the relative sizes often enough so that the child automatically and unconsciously checks. The easiest way is to set up a system where the child checks against letters already written or in place. Therefore practising words that have letters that can be checked against is helpful. Using words that begin and end with the letter "t" is great to start with. E.g. tent, treat, tweet. Having "t" at the beginning gives the child the guidance from the start of the word. And then moving on to other words that contain "t". Then learning to check with letters like "h", "d" - those that have a stick with the other part of the letter at half size. While learning, encourage the child to look at the finished word. Maybe even place a ruler there to check. Not being pedantic about it. It's important to be encouraging and motivating.
Books with dotted lines for the tops of the small letters can be used. However, it is not wise to use them for too long because the child may get so used to the dotted line being there that they just cannot print letters the right size without the dotted line. It is of more use to have a way to check within what is being printed.