Don Hite has a VBS Script To Change A Remote SMS Clients Cache Size, but I wanted to do it with PowerShell:
$Cache = Get-WmiObject -namespace root\ccm\SoftMgmtAgent -class CacheConfig
$Cache.size = 10240
$Cache.InUse = "True"
I added line 3 after trying the size change and the bottom of the Advanced tab on the ConfigMgr control panel was greyed out (on a Windows Vista SP1 client).