Hi there, yes you can add disks with different capacity to a vsan cluster. Just a recomendation from my side would be to keep the same in all the hosts so in the future going to maitenance is not that painfull.
Any documentation relating to this will state that we advise a homogenous configuration across the cluster, however you may also note that most of these resources will state something to the effect of 'per-host homogeneity' as this is the important part (e.g. don't have 3 nodes with 10TB capacity each and 4th node with 30TB capacity) - if you are adding additional disks of larger size to each Disk-Group, this is fine and completely supported. The difference in disks size you noted are not massively varying, a performance impact may be noticeable if for instance you were adding disks twice the size of the current ones as these disks would be storing twice as much data and thus serving twice as many IO operations, however do ensure that the devices you are adding are equal or better in performance spec and type as the ones in use.
"vSAN works best on hosts with uniform configurations"
Relatively old but support statement remains the same: