Assuming that the flow rate of your reactant is kept constant, varying the catalyst bed length and consequently its mass/volume changes the space velocity that you are operating at. Depending on the reaction that you are interested in, the space velocity would be defined as either liquid hourly space velocity (LHSV), gas hourly space velocity (GHSV) or weight hourly space velocity (WHSV). Varying these factors is quite common in heterogeneously catalyzed reactions, and hopefully you should be able to find some data to suit your needs.

Thanks Ville for the response...however, i want to keep the catalyst weight constant..that means changing only the reactor volume. does this effect the reaction?

For a packed bed reactor, the only way to change the packed volume while keeping catalyst weight constant is to change the bed density.  Is that what you mean?

The problem is connected with the effect of axial dispersion (assuming flat velocity profile, which requires quite high ratio Dreactor/Dpellets, min 15 - 20). An analytical expression is given in textbooks, f.ex. see Chemical Reactor Engineering, by O. Levenspiel, Ch.13 The dispersion model. Shorter bed length will give lower conversion, the amount (volume) of the catalyst being the same. At least 7-8 ideal mixing cells in chain is required for approaching the plug flow regime. One pellets layer could be assumed to be one perfect mixing cell.

Now I know what you what.  For homogeneous reactions, the minimum entrance lenth is L/D = 10, so as to rule out axial diffusion as a confounding factor in your data.  For packed beds of catalysts particles having a diameter, dp, the minimum bed length is L >20 dp. 

Thanks mark, Anton for you responses.....

Dear Venkata,

                          If you want to change the catalyst bed length,then the space velocity will change. As a result,the reaction rate will be affected. But if you fix the weight of the catalyst, then the bed height will not change for particular reactor volume. 

Dear Sujoy,

          Thats what Im saying...I want to change the reactor volume keeping the weight of catalyst constant...

The solution is to dilute the catalyst by inert material with the same size of particles (quartz glass, corundum pearls etc. ).

Dear Venkata,

                         If you want to change the reactor volume with fixed catalyst weight then automatically bed height will changed. As a result, the space velocity will changed.

Prepare the catalyst to be attached on a supporting material!

please refer to this study. 

Hydrogen Production from Acetic Acid Steam Reforming over Bimetallic Ni-Co on La2O3 Catalyst-Effect of the Catalyst Dilution

TAT Abdullah, W Nabgan, MJ Kamaruddin, … - Applied Mechanics and Materials, 2014. 

Thank you Ramli Mat, I will check...