WYSIWYG is a word I haven't used in a while, but it does apply to the very simple editing allowed by Lacerte. If it doesn't fit, increase or reduce the font size. or pick a different font. On the the far right you can modify the right and left margins. Use of B (bold), I (Italic), U (underline) will also impact how a letter fits the space. 

I have often found that insufficient and use many space bar entries to get things to line up, ESPECIALLY when trying to include a logo. 

Edit. Save. Print a sample letter. Repeat until you get it right. Once done, that letter format should be good for years to come. Start with the 2017 (or earlier) letter. You can then go to the next year and select Settings > Transfer Prior Year Settings and use the check boxes to pick what you want to bring forward.