While I believe this is user error, I think it would be better not to get an exception. I'll try to find out what the exact input was.

This is the full error:

Failed to call hook 'cmdChatZone' on plugin 'ZoneManager v3.0.12' (FormatException: Input string was not in a correct format.)
  at System.Number.ParseSingle (System.String value, System.Globalization.NumberStyles options, System.Globalization.NumberFormatInfo numfmt) [0x00083] in <437ba245d8404784b9fbab9b439ac908>:0 
  at System.Single.Parse (System.String s, System.Globalization.NumberStyles style, System.Globalization.NumberFormatInfo info) [0x00000] in <437ba245d8404784b9fbab9b439ac908>:0 
  at System.Single.Parse (System.String s, System.IFormatProvider provider) [0x0000c] in <437ba245d8404784b9fbab9b439ac908>:0 
  at System.Convert.ToSingle (System.String value) [0x0000f] in <437ba245d8404784b9fbab9b439ac908>:0 
  at Oxide.Plugins.ZoneManager.UpdateZoneDefinition (Oxide.Plugins.ZoneManager+Zone zone, System.String[] args, BasePlayer player) [0x003f1] in <1b720c3d5bea42919d8c3f732dfd78a1>:0 
  at Oxide.Plugins.ZoneManager.cmdChatZone (BasePlayer player, System.String command, System.String[] args) [0x003f1] in <1b720c3d5bea42919d8c3f732dfd78a1>:0 
  at Oxide.Plugins.ZoneManager.DirectCallHook (System.String name, System.Object& ret, System.Object[] args) [0x03763] in <1b720c3d5bea42919d8c3f732dfd78a1>:0 
  at Oxide.Plugins.CSharpPlugin.InvokeMethod (Oxide.Core.Plugins.HookMethod method, System.Object[] args) [0x00079] in <d09a1f46ca2f4432811bcfe45ad13c7b>:0 
  at Oxide.Core.Plugins.CSPlugin.OnCallHook (System.String name, System.Object[] args) [0x000d8] in <cf88a28c7fb44d36890d85a78331cc9d>:0 
  at Oxide.Core.Plugins.Plugin.CallHook (System.String hook, System.Object[] args) [0x00060] in <cf88a28c7fb44d36890d85a78331cc9d>:0​

The exact command used was this one: 

/zone Size 58 3 20+​

/zone size "58 3 20"

I know the user input is wrong, but it's better practice to not throw an exception due to user error.