what am i doing wrong?

a = [6.935619321	6.044821712	3.134221992	4.03723168	44.35409695	24.3929123	27.83599663	2.360239752	31.63092825	7.189890853	0.639270435	1.404702752	87.21610528	7.956564044	4.976439534	14.98701648	20.04658871	3.245097342	60.65204845	8.473849158	12.88126928	152.5899599	162.5176806	165.3003645	172.9442562	36.1144365	17.27680243	13.61212521	35.61080325	12.91920333	4.060340798	5.183427853	16.82114757	21.53888165	30.87388237	0.500171677	13.90364604	18.71917525	12.20078559	35.42235807	2.360556574	5.146571947	0.688984049	0.571231374	10.98994753	0.610325415	16.73235181	28.38199434	49.11590007	37.24850295	4.96158691	30.82454871	8.600581955	18.60764689	44.97155227	6.267863223	1.29369648	80.35825674	50.37336917	120.402486
    704.7425374	497.8575062	231.0523862	303.9155734	2398.86889	2363.393303	2052.214632	1401.527665	1195.385704	1141.068931	530.1204071	719.0627586	658.2118105	1670.911183	964.4133012	613.2864152	651.5200114	1032.004328	883.933512	926.9718212	669.3164675	855.1756096	877.7346682	873.1162954	852.6209747	870.9879069	2228.003245	835.1785985	495.4477079	2145.984107	980.4355009	1184.624074	354.6949055	444.278561	809.808274	1070.888888	1208.93713	130.9940865	532.878654	692.6306627	828.6450502	628.5106037	748.6055007	786.6259368	885.2770485	933.6825812	851.4713508	1035.56264	884.3207665	1159.39765	1312.298832	1862.273744	271.4662628	562.0994173	347.4607626	2316.859913	485.7127394	399.7244779	1237.155371	1478.748961
    -841.7576842	-535.4401458	-492.1288074	-385.4577355	-2610.862233	-472.6483553	-423.2464952	-2812.842669	-2427.79961	-2235.386912	-1690.107759	-1626.896759	-257.4801241	-3329.131946	-2816.665045	-1218.245293	-728.4260221	-1227.67158	-1397.417942	-1363.334056	-1092.449912	-1208.551645	-1102.110806	-1160.550311	-1007.357699	-1157.746282	-1345.887656	-2679.35944	-1226.720221	-2617.088883	-2738.784324	-1261.888107	-490.367718	-716.0875752	-457.6734926	-538.4369091	-773.0059077	-1498.429036	-1446.717147	-1746.137904	-1961.961576	-134.6617489	-1178.250329	-1312.036393	-341.3551273	-686.7375961	-846.7331219	-954.3391692	-106.2146133	-1271.62049	-1178.210577	-2191.348181	-1944.777367	-641.3823654	-915.4784362	-678.3189285	-2481.942741	-2654.875547	-591.1355787	-437.3663647
    4862.910289	1355.933205	1740.109509	697.266154	10957.77786	20938.3703	3942.430228	312.8555661	7835.792313	2051.919986	179.9962327	27454.99512	20558.87867	-20481.51341	-921.0031122	3468.856724	25559.93643	714.210672	12542.34206	4634.476687	50754.55309	23954.5459	30343.97296	36195.75984	49421.33637	-49706.22754	5029.02627	2461.715672	-74020.41277	-3078.082319	673.2815554	1659.872489	4132.459252	6291.012785	7587.811162	104.6994939	1976.167568	5056.462596	3920.106703	9410.246322	591.1871644	-2207.887378	107.0944364	86.73960378	1894.612108	161.7588786	3030.729399	10092.23733	17197.12127	72133.96032	1163.169661	4682.375674	17574.82174	3891.35099	24981.90974	-2283.196074	261.7046238	11028.39779	17849.93272	21717.11903
    -677.7197919	-319.6740693	-71.42845025	-207.168951	-752.3694602	-877.2136527	-386.8745055	-45.27745973	-526.6557927	-125.4235623	-18.68792141	1975.357795	-490.7437904	-1383.403587	-129.5860491	-163.1735662	-302.2616691	-90.66935992	-602.3054636	-138.6030629	-221.7409722	-1286.176506	-1304.778913	-1301.634913	-1339.894613	-593.4283686	-243.6596197	-216.8395299	-577.2791256	-397.6489027	-67.06907873	-83.43416971	-289.3598963	-266.6904773	-319.4891615	-27.11016096	-2570.197199	-270.8016951	-199.2744951	-580.5057735	-77.01200107	-471.0391248	-36.41676677	-31.92916616	-183.4049563	-82.08328851	-262.4306585	-265.7770551	-351.7104009	-579.6475265	-92.16428519	-332.8595157	-173.2395123	-267.0596267	-493.5616198	-1712.910856	75.48626306	-401.6144294	-415.2271257	-4048.657506];

f1 = (.01 + .1)/2;
f2 = (2 + 4)/2;
f3 = (1 + 5)/2; % centre frequency in Hz
f4 = (5 + 10)/2;
f5 = (.1+1)/2;
Q = []

Q =

     []

for i = 1:size(a(:,i))
    q(:,i) = a(:,i);
    b1 = log(q(:,i));
    b2 = log([f1 f2 f3 f4 f5]');
    p = abs(polyfit(b2, b1, 2));
    n = abs(p(1)); 
    q0 = abs(exp(p(2)));
    Q(:,i) = q0 * [f1 f2 f3 f4 f5]'.^n;
    i = i + 1;
end

Warning: Colon operands must be real scalars. This warning will become an error in a future release.

Answer 1

[Walter Roberson]

size(a(:,i)) is a vector of length 2.

You should use size(a,1)