I should have included this link:
http://www.dheaf.plus.com/warrebeekeeping/cell_size_measurements.htm

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Comb worker cell size measurements

Introduction

The suggestion that bees raised on comb with cells smaller than 
determined by conventional foundation may be better able to cope with 
Varroa1 has attracted the interest of apiculturists and apiologists 
alike. However, a number of experiments reported in peer-reviewed 
journals have failed to demonstrate any overall advantage of small cell 
comb based either on small cell (4.9 mm) foundation or made of 
plastic.2-7 A criticism made of these studies is that they are too 
short-term, the longest lasting being 13 months,4 whereas anecdotal 
evidence accumulated over two decades suggests that a combination of 
withdrawal of acaricides, natural selection and small cell comb needs to 
be applied for several years before a Varroa resistant population of 
bees is established.

An alternative approach has been to let bees build natural comb and 
develop their own preferred cell size range. In a study lasting three 
years, natural comb was compared with comb based on 5.3 mm cell size 
foundation regarding the survival, Varroa mite drop and productivity of 
the respective colonies.8 Perhaps surprisingly, the worker cell size of 
the natural comb colonies was larger (5.4 mm) than that of the 
foundation comb colonies (5.3 mm). No decrease in natural cell size was 
observed over the three years of the study...."

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