laptop and external Monitor 21.5" or 23"

Changed from desktop and 19" monitor to Dell 17" laptop few months ago. Lately noticing some eyestrain, tiredness etc. by days end. Spend about 7-8 hrs in front of screen off & on over the day. Wondering if need to go back to large monitor. Main progs used are
Excel
Word
MS Outlook
Revenue offline application(ROS)
Big Red Book
Thesaurus Payroll
Have been looking at these two from dellhttp://accessories.euro.dell.com/sna/productcompare.aspx?c=ie&category_id=4009&cs=iebsdt1&l=en&nf=33811~0~162929&s=bsd&prods=308725,309074

Just wondering, is 23" too big, does anyone use 21.5 or 23 for excel etc. and how do they find it.
Thanks.

I use a 23" Graham and it is fine, certainly not too big.

Thanks for that, was just wondering if it would be like being at the cinema !.

Not quite! But it is certainly easier on the eyes!

Thanks for that again, 23" ordered.