Use the plug() method to initialize the DataSourceCache plugin and pass in the configuration value max to set the maximum size.

YUI().use("datasource", "dataschema", "cache", function (Y) {
    var callback = {
            success: function (e) { /* output to screen */ },
            failure: function (e) { /* output to screen */ }
        },

        myDataSource = new Y.DataSource.Get({
            source: "https://api.github.com/users/",

            // this is only needed because the query appends the url
            // rather than the url's query params
            generateRequestCallback: function (guid) {
                return '/repos?callback=YUI.Env.DataSource.callbacks.' + guid;
            }
        }),

    myDataSource.plug(Y.Plugin.DataSourceJSONSchema, {
        schema: {
            resultListLocator: "data",
            resultFields: ["name"]
        }
    });

    myDataSource.plug(Y.Plugin.DataSourceCache, { max: 3 });

    // Adds to cache
    myDataSource.sendRequest({
        request : "lsmith",
        callback: callback
    });

    // Adds to cache
    myDataSource.sendRequest({
        request : "davglass",
        callback: callback
    });

    // Retrieves from cache
    myDataSource.sendRequest({
        request : "lsmith",
        callback: callback
    });
});

Full Example Source Listing

<form id="demo" action="http://search.yahoo.com/search">
<h6>Look up github repositories by username (e.g., davglass, lsmith or rgrove):</h6>
<input type="input" id="demo_input_query" name="p">
<input type="submit" id="demo_query_retrieve" value="Retrieve data">
<input type="button" id="demo_cache_clear" value="Clear cache">
<div id="demo_output_response" class="output"></div>
</form>

<script type="text/javascript">
YUI().use("json-stringify","node", "datasource-get", "datasource-jsonschema", "datasource-cache", "datatype-date", function (Y) {
var output = Y.one("#demo_output_response"),
    source = "remote source",

    myDataSource = new Y.DataSource.Get({
        source:"https://api.github.com/users/",
        generateRequestCallback: function (guid) {
            return '/repos?callback=YUI.Env.DataSource.callbacks.' + guid;
        }
    }),

    callback = {
        success: function(e){
            var when = Y.DataType.Date.format(new Date(), {format:"%F %r"}),
                data = Y.JSON.stringify(e.response, null, 2);

            output.setHTML(
                "<p>[" + when + "] Retrieved from " +
                "<strong>" + source + "</strong></p>" +
                "<pre>" +
                    data.replace(/&/g,"&amp;")
                        .replace(/</g,"&lt;")
                        .replace(/>/g,"&gt;") +
                "</pre>");
        },
        failure: function(e){
            var when = Y.DataType.Date.format(new Date(), {format:"%F %r"}),
                message = /fields retrieval/.test(e.error.message) ?
                    "User not found" : e.error.message;

            output.setHTML(
                "<p>[" + when + "] Could not retrieve data: " +
                    "<em>" + message + "</em>" +
                "</p>");
        }
    };

myDataSource.plug(Y.Plugin.DataSourceJSONSchema, {
    schema: {
        resultListLocator: "data",
        resultFields: ["name"]
    }
});

myDataSource.plug(Y.Plugin.DataSourceCache, { max: 3 });
myDataSource.cache.on("retrieve", function(){
    source = "cache";
});

Y.one("#demo_cache_clear").on("click", function(){
    var when = Y.DataType.Date.format(new Date(), {format:"%F %r"});

    myDataSource.cache.flush();
    output.setHTML("<p>[" + when + "] Cache cleared.</p>");
});

Y.on("submit", function(e){
    e.halt();
    var query = encodeURIComponent(
                    Y.one("#demo_input_query")
                        .get("value")
                        .replace(/"/g,'\\"')
                        .replace(/\W/g, ''));

    if(query) {
        source = "remote source";
        myDataSource.sendRequest({
            request:query,
            callback:callback
        });
    } else {
        output.setHTML("<p>Please enter a query.</p>");
    }
}, "#demo");
});
</script>