It would be a better idea not to use deprecated tags like
<font>
especially as it is no longer supported in XHTML 1.0 Strict, and instead use inline style elements like so
$name_exist = "<p style=\"font-size:1.1em ;\">$BloomName is a Registered Cultivar. Please Rename your Cultivar.</p>";
echo $name_exist;
it's also poor practise to use fixed size fonts, hence my use of the em size, you could also use % sizes, both of which would give you a size proportionate to your base font size.