I agree that if Veeam Monitor (or probably more appropriate for Veeam Reporter) could provide this info it would be nice, but here are some third party option for you:
The virtualization community has provided a good script to monitor daily rate of data change from CBT. This can be used for VMs that have not already been backed up yet by Veeam and thus do not appear in the Backup job > Properties section of Veeam BaR (but we're clear that all VMs should be backed up by Veeam, right?!
How much bandwidth for offsite VM replication? - http://jpaul.me/?p=1316
Although not an official Veeam tool, Veeam employee Ricky El Qasem created a tool that could help
Replication Bandwidth Calculator - http://read.virtualizeplanet.com/?p=91
You can also use a formula to "guestimate" how much space you will need for replication jobs. You need to put in the expected values for various job criteria.
the following is copied from viewtopic.php?f=2&t=3498#p14246
... the formulas we use for disk space estimation are the following:
Backup size = C * (F*Data + R*D*Data)
Replica size = Data + C*R*D*Data
Data = sum of processed VMs size (actually used, not provisioned)
C = average compression/dedupe ratio (depends on too many factors, compression and dedupe can be very high, but we use 50% - worst case)
F = number of full backups in retention policy (1, unless backup mode with periodic fulls is used)
R = number of rollbacks (or increments) according to retention policy (14 by default)
D = average amount of VM disk changes between cycles in percent (we use 10% right now, but will change it to 5% in v5 based on feedback... reportedly for most VMs it is just 1-2%, but active Exchange and SQL can be up to 10-20% due to transaction logs activity - so 5% seems to be good average)