Your question has actually nothing to do with spritefont, but with expressions you can use to manipulate strings, values and floats.
First I wanted to mention the FormatDecimal expression to you, but then I realized you wanted to manipulate the number of characters before and after the dot, maybe even with letters and not only numbers like in your example.
So you'll need Gettoken, it's a system expression. Take a look here:
About the most important stuff to know about Construct.
So lets say you have a string "dadadudu.muhaha" and want to limit it to 4 characters before the dot and 2 characters after:
Gettoken can take your string apart by using a delimiter character. In this example it's obviously ".". So when you write Gettoken("dadadudu.muhaha",1,"."), it will return the first token, which would be "dadadudu". Now this is enclosed in a left expression, which returns the defined number of characters from the left. Left("dadadudu",4) gives you "dada".
So by getting the two tokens, limiting them to the wanted amount of characters and putting a dot in between, you get the string you want. In case you need to understand more about these expressions just use the link above. Like I said, it's essential to know about the system expressions!