[jhowell]

I haven't used this myself, but here is the information on dropcaps from the Amazon Kindle Publishing Guidelines version 2015.3.

Elements such as drop caps should be specified using percentages or relative units (positive or negative) instead of fixed values such as points and pixels. (Example: drop caps: Use font-size: 300%). The top of the drop cap should be aligned with the body text. To create drop caps, Amazon recommends using the following sample CSS:

Example

p.para {
 font-family: "Times New Roman";
 font-size: 4em;
 margin-bottom: 0;
 margin-top: 0;
 text-align: justify;
 text-indent: 0;
}

@media amzn-kf8
{
 span.dropcaps
 {
 font-weight:normal;
 font-size:320%;
 float:left;
 margin-top:-0.3225em;
 margin-bottom:-0.3245em;
 }
}

@media amzn-mobi
{
 span.dropcaps
 {
 font-size:3em;
 font-weight: bold;
 }
}

<p class="para"><span class="dropcaps">T</span>here is a sample

To verify that the drop caps display as intended, test the book as described in section 9.1, Testing Kindle Books.

ETA: I realize that this probably won't be very helpful. There are several values that are part of this example/recommendation with no indication of how they were derived or under what circumstances they would need to be changed.