This work is concerned with the estimation problem of linear model when thesample size is extremely large and the data dimension can vary with the samplesize. In this setting, the least square estimator based on the full data is not feasiblewith limited computational resources. Many existing methods for this problem arebased on the sketching technique which uses the sketched data to perform leastsquare estimation. We derive fine-grained lower bounds of the conditional meansquared error for sketching methods. For sampling methods, our lower boundprovides an attainable optimal convergence rate. Our result implies that when thedimension is large, there is hardly a sampling method can have a faster convergencerate than the uniform sampling method. To achieve a better statistical performance,we propose a new sketching method based on data averaging. The proposedmethod reduces the original data to a few averaged observations. These averagedobservations still satisfy the linear model and are used to estimate the regressioncoefficients. The asymptotic behavior of the proposed estimation procedure isstudied. Our theoretical results show that the proposed method can achieve afaster convergence rate than the optimal convergence rate for sampling methods.Theoretical and numerical results show that the proposed estimator has goodstatistical performance as well as low computational cost.