If your your word processing package has the facility, you could add a border round the wording. Print out the sheet onto thin printer card or heavy paper.

Then cut a 6 cm strip off the right hand side using a ruler and craft knife (or deckled edged scissors, available from craft shops, for a more decorative effect).

  Score line down either side of the text with a blunt knife and then  fold both the shorter edges into the middle, allowing an overlap of about 3 cm (this should then fit into an standard envelope). This is easier than having an edge-to-edge join down the middle which can be tricky to get exactly right. Take 50cm of ribbon in the wedding theme colour and fix the middle of it to the outer  back of the invitation with a spot of glue then tie the ends in a bow at the front.