SharePoint Designer 2010 Errors were found when compiling the workflow. The workflow files were saved but cannot be run. Unexpected error on server associating the workflow.

First, make sure SharePoint Farm is at February CU or later build, the following property will be made available:

UserDefinedWorkflowMaximumComplexity

Please refer to the link below for more information:

http://msdn.microsoft.com/en-us/library/microsoft.sharepoint.administration.spwebapplication.userdefinedworkflowmaximumcomplexity.aspx

By installing a CU that includes the server February 2011 CU, SharePoint administrators are now able to prevent compilation of workflows whose compilation would affect the performance of the farm. The default value of the property is 7000, but can be changed based on the needs and profile of the SharePoint farm.

The following PowerShell script can be used to adjust the value per SPWebApplication:

[System.Reflection.Assembly]::LoadWithPartialName("Microsoft.SharePoint")

$new_limit = 5000;

$webapp = [Microsoft.SharePoint.Administration.SPWebApplication]::Lookup(http://WebAppURL)

$webapp.UserDefinedWorkflowMaximumComplexity = $new_limit

$webapp.Update()

To give context to the default 7,000 value a Start Approval Process action contains 1176 nodes, a Start Feedback Process action contains 1010 and an empty Start Custom Task Process contains 13 nodes.

Please note, up to certain number, roughly 10000, the compliation will be restricted by Microsoft .NET 3.0 workflow complier so the workflow size still has a ceiling.
At that point, the larger workflows need to be restructured to either have more reusable pieces, or be broken out into smaller individual small workflows.

Some ways in which the complexity of larger workflows can be reduced include:

• Use light weight workflow actions to replace Start Approval Process action, for instance, Start Custom Task Process action or Collect Data from a User
• Add new task process variables only as they’re needed
• When the task approvers (or task email) need to change based on conditions in the workflow, use a conditional block to save the task approver (or email contents) in a local variable and then use a single Approval Process action, rather than having a separate Approval Process actions with hardcoded approvers/email contents in each branch of the conditional block
• When using the small workflows approach, keep the second workflow into a paused state by using an action like Wait for Field Change. In the first workflow, once the Start Approval actions have been completed the workflow can modify a column which the second workflow is monitoring and then it will come out of the paused state and go on