Description of problem:
Once in custom partitioning, I blanked out the size value for the root partition (i was using normal partitoning, not LVM), and clicked update settings. 


Version-Release number of selected component:
anaconda-20.10-1

The following was filed automatically by anaconda:
anaconda 20.10-1 exception report
Traceback (most recent call first):
  File "/usr/lib/python2.7/site-packages/blivet/size.py", line 80, in _parseSpec
    raise ValueError("invalid size specification", spec)
  File "/usr/lib/python2.7/site-packages/blivet/size.py", line 138, in __new__
    self = Decimal.__new__(cls, value=_parseSpec(spec))
  File "/usr/lib64/python2.7/site-packages/pyanaconda/ui/gui/spokes/custom.py", line 156, in size_from_entry
    size = Size(spec=size_text)
  File "/usr/lib64/python2.7/site-packages/pyanaconda/ui/gui/spokes/custom.py", line 1137, in _save_right_side
    size = size_from_entry(self._sizeEntry)
  File "/usr/lib64/python2.7/site-packages/pyanaconda/ui/gui/spokes/custom.py", line 2788, in on_apply_clicked
    self._save_right_side(self._current_selector)
ValueError: ('invalid size specification', 'MB')

Additional info:
cmdline:        /usr/bin/python  /sbin/anaconda
cmdline_file:   initrd=initrd.img inst.stage2=hd:LABEL=Fedora\x2020-Alpha-TC4\x20x86_64 quiet BOOT_IMAGE=vmlinuz 
executable:     /sbin/anaconda
hashmarkername: anaconda
kernel:         3.11.0-3.fc20.x86_64
product:        Fedora
release:        Cannot get release name.
type:           anaconda
version:        20-Alpha-TC4