I think this issue was first reported in this comment of JENKINS-50470 but since it looks like a similar but different problem I'll try to expand on it here.

When executing a Groovy system script from a file in the job workspace, invoking the size method on an ArrayList without parentheses the script security plugin raises the following error

ERROR: Build step failed with exception
{{ org.jenkinsci.plugins.scriptsecurity.sandbox.RejectedAccessException: No such field found: field java.lang.String size}}
{{ at org.jenkinsci.plugins.scriptsecurity.sandbox.groovy.SandboxInterceptor.unclassifiedField(SandboxInterceptor.java:419)}}
{{ at org.jenkinsci.plugins.scriptsecurity.sandbox.groovy.SandboxInterceptor.onGetProperty(SandboxInterceptor.java:403)}}
{{ at org.kohsuke.groovy.sandbox.impl.Checker$6.call(Checker.java:288)}}
{{ at org.kohsuke.groovy.sandbox.impl.Checker.checkedGetProperty(Checker.java:292)}}
{{ at org.kohsuke.groovy.sandbox.impl.Checker.checkedGetProperty(Checker.java:255)}}
{{ at org.kohsuke.groovy.sandbox.impl.Checker$checkedGetProperty$0.callStatic(Unknown Source)}}
{{ at org.codehaus.groovy.runtime.callsite.CallSiteArray.defaultCallStatic(CallSiteArray.java:56)}}
{{ at org.codehaus.groovy.runtime.callsite.AbstractCallSite.callStatic(AbstractCallSite.java:194)}}
{{ at org.codehaus.groovy.runtime.callsite.AbstractCallSite.callStatic(AbstractCallSite.java:230)}}
{{ at Script1.run(Script1.groovy:3)}}
{{ at org.jenkinsci.plugins.scriptsecurity.sandbox.groovy.GroovySandbox.run(GroovySandbox.java:141)}}
{{ at org.jenkinsci.plugins.scriptsecurity.sandbox.groovy.SecureGroovyScript.evaluate(SecureGroovyScript.java:333)}}
{{ at hudson.plugins.groovy.SystemGroovy.run(SystemGroovy.java:95)}}
{{ at hudson.plugins.groovy.SystemGroovy.perform(SystemGroovy.java:59)}}
{{ at hudson.tasks.BuildStepMonitor$1.perform(BuildStepMonitor.java:20)}}
{{ at hudson.model.AbstractBuild$AbstractBuildExecution.perform(AbstractBuild.java:744)}}
{{ at hudson.model.Build$BuildExecution.build(Build.java:206)}}
{{ at hudson.model.Build$BuildExecution.doRun(Build.java:163)}}
{{ at hudson.model.AbstractBuild$AbstractBuildExecution.run(AbstractBuild.java:504)}}
{{ at hudson.model.Run.execute(Run.java:1810)}}
{{ at hudson.model.FreeStyleBuild.run(FreeStyleBuild.java:43)}}
{{ at hudson.model.ResourceController.execute(ResourceController.java:97)}}
{{ at hudson.model.Executor.run(Executor.java:429)}}
{{ Build step 'Execute system Groovy script' marked build as failure}}

The script in question is

list_of_str = []
list_of_str = ['foobar', 'barbaz', 'bazqux']
println "Size " + list_of_str.size

As in the comment originally reporting the issue. Adding parentheses to list_of_str.size() works around the issue.

Minimal job configurations that reproduce the failing case and the workaround are attached as failing_job.xml and passing_job.xml respectively. The scriptApproval.xml required for the passing job to actually pass is also attached. I've also uploaded a Docker image that may make reproducing and testing this issue more expedient. The instructions for the Docker image's use and the source for its creation are at https://github.com/nuclearsandwich/docker-jenkins-script-security-repro