It is actually same for all 3: Internet Explorer, Fire Fox and Chrome. And it is 16px.

 It's sounds too big, but it is, here is the code that we are using:

Regular<span style="font-size:10px;">font-size:10</span><br />

Regular<span style="font-size:11px;">font-size:11</span><br />

Regular<span style="font-size:12px;">font-size:12</span><br />

Regular<span style="font-size:13px;">font-size:13</span><br />

Regular<span style="font-size:14px;">font-size:14</span><br />

Regular<span style="font-size:15px;">font-size:15</span><br />

Regular<span style="font-size:16px;">font-size:16</span><br />

Regular<span style="font-size:17px;">font-size:17</span><br />

Regular<span style="font-size:18px;">font-size:18</span><br />

Regular<span style="font-size:19px;">font-size:19</span><br />

Here is the output of the code: