There is a nice little trick for that . Have the first < be the size of the font you want, then mark the rest of the text including finishing > and set it to font size 1. Then your object will be a lot smaller and you can make the size you want.
While I use the trick Johan mentions, I DO use the Inspector to resize Merge Fields, as well.
Holy cow! I can't believe I never thought to use the inspector to reduce the width of a merge object and that it would retain the current height when doing this! I always used the trick that Johan mentioned.