Hi, stitching half stitch on 14ct wouldn't change the size, just the length of time to complete the stitching. DMC actually had some information on this style of stitching that I think will help you out: http://www.dmc.com/majic/pageServer/0801010321/en_US/Half-Cross-Stitch.html
It does mention using Pearl Cotton but you could use regular floss. I think 3-4 strands should give you nice coverage. I would suggest doing a little sampler of different strands numbers to see which one works best for you.
I hope this helps. Also, I hope to see some pics of the project.