Steps to reproduce:

0. Open new OOo document
1. Menu 'Tools -> Option -> WRITER -> General - Measurement Unit', select "mm"
2. Create new Form Field "Text Field", span size app 160 x 220mm
  (app. (complete page, using mouse)
3. Click on Text Field, right click, context menu 'Position and Size'
   expected: width / height panes should show something like 160 x 220mm
   actual: 99,99mm / 99,99mm

Addtitional effect (IMHO related):
10. Open new OOo document
11. if necessary, menu 
    'Tools -> Option -> WRITER -> General - Measurement Unit', select "mm"
12. Create new Form Field "Text Field", span size app 160 x 10mm 
   (app. (complete page, using mouse)
13. Right Click on Text Field, context menu 'Position and Size'
    modify height to 20mm
    expected: (only) height should be modified
    actual: height modified as expected, width shrinks to 99,99mm

Inputs > 99,99 mm will be accepted and proceeded correctly.

Works fine with unit "cm"

Same with "Ooo Dev 3.2.0 multilingual version English UI WIN XP: [DEV300m53
(Build 9412)]" and "Ooo 3.1.0 WIN XP multilingual version German UI activated 
[OOO310m11 (Build 9399)]"