Something that I see a lot as a reviewer is a phrase something like this:
… about the size of a warbler...
or
… about the size of a sparrow...
and similar entries. These sorts of comparisons are fine with some groups, but these two groups have fairly large size variation from smallest to largest, so the phrase does not really work as a good descriptor of size.
As example, I start with the warblers, that is, the New World warblers (family Parulidae). Now that Yellow-breasted Chat has been moved to its own family (Icteriidae; not to be confused with the New World blackbirds, Icteridae), the size variation within the family has been greatly decreased. However, the range of sizes of ABA-Area warbler species (using the length estimates provided in the Sibley Guide, 2nd ed. as a gross indicator) is 4.25-6 inches. Yes, that 1.75 inches from smallest to largest is a pretty small absolute difference. However, as a relative difference, it's huge: >41% of the smallest length.
If we take that 41.2% size difference in warblers and scale that difference to something larger, like humans, then that relative size difference becomes more apparent. Using data from this website, the average heights of human males in various countries ranges from 160 cm (5'3") in Timor to 182 cm (just less than 6'0") in The Netherlands. That difference of just about nine inches is 13.75% of the smallest average height. As you can see, the relative difference in size across warblers is MUCH larger, in fact, just about three times larger!
So, is your bird the size of a Lucy's Warbler (4.25"), the size of an Ovenbird (6"), or of something somewhere betwixt the two, say something common like a Yellow Warbler (5")?
The sparrows provide another, even more-pointed example. The range of lengths of ABA-Area sparrow species (family Passerellidae; not to be confused with the Old World sparrows, such as House Sparrow, family Passeridae) is 5-9.5"! That is a difference over the smallest species of 90%, more than double the difference among warblers! Of course, if we remove the towhees from consideration, that difference changes to ONLY 50% (5-7.5").
So, is your bird the size of a Grasshopper Sparrow (5"), a Vesper Sparrow (6.25"), a Fox Sparrow (7.5"), a Spotted Towhee (8.5"), or an Abert's Towhee (9.5")?
The take-home message is to be more precise in your size estimates where possible. That is because we reviewers don't know what you mean if you compare something's size to that of a large, variable family of birds. Imagine comparing some bird to a family like Accipitridae, which ranges in size in the ABA Area from male Sharp-shinned Hawk to female Bald Eagle.