There is another way which will work even if you've already done a full backup.....
1. Do your first backup (or second or third, as long as the sparsebundle file has not already increased to a size greater than the limit you are setting out to achieve.

2. Temporarily switch off time machine (so it doesn't try and backup while you're doing this.

3. A sparsebundle is really a folder which Finder pretends is file (a bit like the programs in your applications folder). The maximum size for a sparsebundle file is stored in an xml file named Info.plist which lives inside the .sparsebundle folder.

4. If your time capsule disk is mounted (double click on it in finder) you can very easily access this from the command line.
a. Open the terminal (In Applications==>Utilities
b. type in
sudo nano /Volumes/time-capsule/tim.sparsebundle/Info.plist
[ yours will have a different volume and sparsebundle name obviously ]

c. If you have a 1TB time capsule the file will look something like this

<?xml version="1.0" encoding="UTF-8"?>

<plist>
<dict>
<key>CFBundleInfoDictionaryVersion</key>
<string>6.0</string>
<key>band-size</key>
<integer>8388608</integer>
<key>bundle-backingstore-version</key>
<integer>1</integer>
<key>diskimage-bundle-type</key>
<string>com.apple.diskimage.sparsebundle</string>
<key>size</key>
<integer>998057361408</integer>
</dict>
</plist>

d. change the final number (998057361408 in this example) to a smaller number
(409600032768 will make it limited to 400GB)

e. press control-o to save and then control-x to exit
f. copy this file to its bckup counterpart... type in:
sudo cp /Volumes/time-capsule/tim.sparsebundle/Info.plist /Volumes/time-capsule/tim.sparsebundle/Info.bckup
e. close the terminal.

5. Turn on time machine again and relax.